Add comprehensive XAML "theming" functionality

As I mentioned in https://github.com/microsoft/terminal/issues/3322#issuecomment-546391285, we've been dancing around the idea of "XAML theming" for a while in a bunch of issues, but never had one place to track all the requested functionality. This will serve as the master thread for those requests.

It looks to me like we've danced around the idea in #1963, #1337, #3061/#3062, #2994, but never had a comprehensive answer.

Ideally, these are something that's more powerful than just setting the "color scheme". This would control sizing, coloration of UI elements of the app itself, not just the colors of the terminal contents. Consider things like themes in VsCode, Sublime Text, where there are schemes that can control the colorization of the buffer, and themes that can change the appearance of the app itself.

UI elements to be able to control:

• #3061 Pane Border colors (both the background, and the "focused" color)
• #3063 Pane border width
• #702 👀 Tab Background color
  • also: #1337
  • also: #2994
• #13684
• #3774 👀 Tab row BG color
  • #1963 👀 users should be able to set it to the system accent color
  • Users want the tab row to match the BG color of the focused pane/terminal. This is the inverse of #702
• #3335 👀 Feature Request: Setting to hide/remove close ("x") button from tabs
• #4862 👀 Support using different colors in the titlebar for focused/unfocused windows
• #5911 Support for compact, default, and touch friendly tab row sizes
• #7213
  • Also used for setting the corner radius
  • Also tracking "get rid of the bottom, outwards corner radius"
• #16114
• #19369

Other ideas:

• #5155 Tab row height
• Tab row font size, font face
• Margin between tabs?
• Padding within the tab?
• Control colors for light vs dark vs high-contrast modes
• Enable/disable a shadow underneath the tab row, between tabs and content
  • more relevant after #12916 which added the shadow to the whole row
• Enable/disable a shadow cast by terminals on pane borders or a shadow cast by pane borders on Terminal panes
• Similarly to the tabs, styling the Status Bar (#3459)
  • Maybe enable it to have the same color as the active TermControl, causing the same "seamless" effect (https://github.com/microsoft/terminal/issues/3459#issuecomment-550501577)
  • Change font size, face, colors
  • Control the borders on the status bar - no top border would give the impression it's "seamless"
• Enable hiding the tab icon altogether (#8157)
• Enable forcing tab icons to monochrome
• Force a colorscheme? This seems weird, and probably not what people want.
• Allow color fields to refer back to the active color scheme (like cursorColor = yellow) (#7522)
• Please provide a new setting: verticalScrollBarWidth #9218
• Light mode in settings UI only #9231
• #13991
• #12632

Currently we don't have all that many UI elements, but in a hypothetical world with a command palette (#2046) and a search box (#605), there'll be even more UI elements to be able to control.

I know @cinnamon-msft had some mockups of "themes".

I imagine that these would be something that would be easier to control with XAML resources somehow, though I'm not sure how technically possible it would be to have the user specify a XAML file and have us load it into our resources at runtime. But that might be an option to pursue as an alternative to adding tons of new settings that will need to be parsed and applied manually at runtime.

Potential solution design:

{
    "applicationTheme": "My Boxy Theme",
    "themes": [
        {
            "name": "My Boxy Theme",
            "requestedTheme": "dark",
            "tab.radius": 0,
            "tab.padding": 5,
            "tab.background": "terminalBackground",
            "tab.textColor": "key:SystemAccentColorLight3",
            "tab.icon": "monochrome",
            "tab.closeButton": "hidden",
            "tabRow.background": "accent",
            "tabRow.shadows": false
        },
        {
            "name": "My small light theme",
            "tabBackground": "#80ff0000",
            "tabRowBackground": "#ffffffff",
            "tabHeight": 16,
            "requestedTheme": "light",
            "colorSheme": "Solarized Light",
            "tabIcon": "hidden",
            "tabCloseButton": "hover"
        }
    ]
}

Terminal should be able to look beautiful, and also like this:

Followup flow chart:

flowchart TD
    subgraph Theming
        id_b[tabRow.background]
        id_c[ themeColor:accent ]
        id_d[ themeColor:terminalBackground ]

        %% id1[Theming #12992]
    end

    id2[tab.background<br>#702<br>PR #13178]
    id3[tabRow.inactiveBackground<br>#4862<br>PR 13049]
    id4(tab.inactiveBackground<br>#4862)
    tabclose[tab.closeButton<br>#3335<br>PR #13348]

    id15{{Terminal v1.16}}

    %% Initial theming support
    %% id_a-->id1
    %% id_b-->id1
    %% id_c-->id1
    %% id_d-->id1
    
    %% 1.16 features
    Theming --> id3
    Theming --> id2
    id2 --> id4
    id3 --> id15
    id4 --> id15
    id2 --> tabclose  --> id15
    Theming --> id_transparent(Transparent titlebars<br>window.useMica<br>#10509)
    Theming --> id_acrylic_tabRow(Acrylic titlebar) --> id15


    %% Etc features
    subgraph Pane
        direction TB
        pane.borderColor(pane.borderColor<br>#3061)
        pane.inactiveBorderColor(pane.inactiveBorderColor)
        pane.borderWidth(pane.borderWidth<br>#3063)
        pane.borderColor --> pane.inactiveBorderColor
    end
    subgraph Tab
        direction TB
        tabcorners(tab.corners<br>#7213)
    end
    subgraph Window
        direction TB
        window.background(window.background<br>#16114)
        window.frameColor(window.frameColor) --> rainbow(themeColor:rainbow<br>#12950)
    end

    id15 --> Pane
    id15 --> Tab
    id15 --> Window
    id15 --> lightDark(OS-theme sensitive theming)
    click id4 "http://www.github.com/microsoft/terminal/issues/4862"

No implementation files, tests, or entry points are named. Start by reviewing the related issues listed in the thread and the proposed theme JSON/XAML-resource design, then resolve the open questions and narrow the requested controls into an actionable scope with clear completion criteria.