[Megathread] Fullscreen & Focus Mode follow-up work

Taken from https://github.com/microsoft/terminal/issues/531#issuecomment-531046569

I'd suggest that this is broken into 3 implementation steps:

1. Support fullscreen mode, with always hidden tabs
2. Add a setting to have tabs always visible in fullscreen mode
3. Enable "revealing" tabs on mouse hover near the top

Step 3 has some implementation details that are missing, like: on tabs reveal, should the tabs:

• cover the terminal content, obscuring the top rows, but leaving the rest of the window unchanged
• cause the terminal to resize
• shift the terminal content down, obscuring the bottom rows of the terminal, but not forcing a resize?

Now that #3408 is merged (closing #531), we need some place to track the follow-up tasks I had outlined before:

Focus mode and Fullscreen share a lot of commonalities, so I'm grouping them into one thread for easy of x-linking.

Fullscreen

• Support fullscreen mode, with always hidden tabs
• #11130 - Add a setting to have tabs always visible in fullscreen mode
  • "showTabsInFullscreen": true|"always"
• #5677 - Enable "revealing" tabs on mouse hover near the top
  • "showTabsInFullscreen": "hover"
• If a launch full screen option is there, when the full screened app launches, I think the TabView should be visible for a few seconds before sliding out of view. (from @mdtuak in https://github.com/microsoft/terminal/issues/288#issuecomment-551950312)
  • this is dependent upon adding fullscreen to launchMode, tracked in #288
• #4000 It would be nice to have a kind of popup with a tab name when you switch tabs with CTRL+TAB in fullscreen mode.
  • Let's add a setting for this as well, this sounds like a not so terrible idea.
  • This is the "advanced tab switcher"

Focus Mode

• #539
  • Not necessarily Focus Mode related, but certainly relevant
• #10730
• #7210
• #12959

Start by reviewing the linked follow-up issues #11130, #5677, #288, #539, #7210, and #12959, along with the completed items in this checklist. The issue does not name source files or tests, and completion requires resolving multiple fullscreen and focus-mode tasks rather than one defined change.