Stick profile appearance settings into Theme objects for easier sharing of settings

A fever dream I had:
 
`theme.profile { an appearance }`, and we use that as the "hardcoded default" (or, I suppose the first layer).

That would let folks put the WHOLE appearance of the terminal into a theme, for sharing. background images, background colors, color schemes, fonts, you name it. Now it's just one blob you put into the settings.

Settings from dynamic profiles, `profiles.defaults`, and profile settings would all still take precedence.

The issue names no files, tests, or entry points. Begin by clarifying the Theme object and appearance-settings boundaries, including how shared theme values interact with dynamic profiles, profiles.defaults, and profile settings; done would mean the precedence rules and full appearance scope are defined well enough to implement.

Written by the indexing model from the issue text.