Improve focus handling

[PoignardAzur]

Group issue for focus-related problems. Needs to be fleshed out.

For the roadmap:

We should improve tab focus in Masonry. Right now tab focus has some bugs, doesn't cover buttons, isn't updated by clicks, and uses architecture inherited from Druid that could be strongly simplified once other architectural changes are completed.

[mwcampbell]

I just want to mention that this is important for accessibility, particularly for blind users. This isn't strictly necessary for screen reader accessibility, because screen readers have commands to navigate the window and perform the accessibility action corresponding to a click, and using those commands is particularly common with VoiceOver on macOS. But on Windows at least, blind users are accustomed to using keyboard focus, i.e. tabbing and activating buttons or check boxes with Space.