Full contextual theming

[viridia]

Third time's the charm, maybe? This now implements full-on contextual theming.

Token lookup now happens in two stages:

A theme token is converted into a semantic token, and then the combination of semantic token + theme context is used to look up a color. The set of semantic tokens is much smaller than the set of theme tokens.

The theme context is an enum which indicates the surface layer of the container holding the widget.

@alice-i-cecile @gagnus

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[viridia]

I ended up completely re-writing the previous version ("limited contextual theming") but re-used the same PR instead of creating a new one (partially by accident). In any case, I think this is ready for review.

@gagnus I'd be interested in your comments; as this moves somewhat nearer to your "slots" idea. I have not attempted to implement an algorithmic theme generator (I'll leave that to you or someone else), I'm just trying to put in the infrastructure for contextual theming and semantic tokens.

I have continued to use alpha colors in a few places - this is primarily for ease of migration on my part. There's no longer an "alpha hack" for sliders and text input backgrounds. Alpha is mainly used for disabled controls, because this is the specification given on the figma doc. However, with some effort, the number of alpha colors could be reduced even further if desired (although there are a few things, like focus rects, which depend on alpha).

[viridia]

Here's a screenshot of what it looks like. There are some subtle differences in color assignments - it was hard to get checkboxes to have the exact same shades while fitting all the colors into a consistent framework. Other changes are intentional: You can see that the text input fields now have context-sensitive colors: the fill color for the input fields in the group container are lighter than the fill color for the input field on the default background:

Overall, this is actually closer to the current figma mock than what's in mainline right now. (Although looking at it right now I see some bugs in the toggle switches. Note that the figma design for the toggles is quite pedestrian IMHO.)

Note that the feathers gallery color widgets are getting cut off because people have been stuffing more widgets into the first column and pushing the other items down. This needs to be fixed, but not in this PR.

We also need a new demo which demonstrates all of the context-sensitive widgets shown against the various background choices. I'm thinking of something that has 4 columns: window, pane, subpane, and group.