Add 24-bit theme support

[rht]

This is a tracking issue for urwid/urwid#284. Once merged, I will extend gruvbox to 24-bit, and also this will make adding other true color themes easier.

[tonycpsu]

@rht : I've merged the PR. Can't say when a release will go out, but you can at least point to urwid master now.

[rht]

Finally!

[Ezio-Sarthak]

This would be as easy as adding a flag 2**24 in --color-depth argument.

We could also discuss here adding 88 too, though the default dark view for it doesn't seem readable at all :)

All the options for urwid's set_terminal_properties could be seen here

[neiljp]

Given the need for color-depth/theme compatibility after #998, I don''t mind if we leave this open to resolve the literal issue title, or close and break up into smaller issues - but I'd definitely like something open to track the incomplete support we're at right now, useful as it is already as an intermediate state 👍

[neiljp]

I'm going to close this since we now support 24-bit features in themes, even if not every built-in theme has a 24-bit-specific set of colors - ie. we need to add those values to DefaultColor if we want them to be more specific (as of #1051). The latter PR means we also don't need 24-bit-specific themes (ie. gruvbox + gruvbox24), which makes things much simpler.

See #1069 for tracking follow-up.