Remaining tasks after category colors PR

Continuing on #254

- [ ] Make more visualizations use the new coloring logic
- Summary
- [x] Windows, apps
- [x] Editor, browser, etc.
- [ ] Sunburst clock
- [ ] Timeline (https://github.com/ActivityWatch/activitywatch/issues/573)
- [x] Figure out how the upgrade process will work (where people have existing categories without `data.color` set)
- Messed this up in the release, but fixed now (https://github.com/ActivityWatch/activitywatch/issues/612#issuecomment-865032648)
- [x] Which colors should be set as default for the default categories?
- [x] Add a color picker/palette (fixed in https://github.com/ActivityWatch/aw-webui/pull/273)
- [ ] Fix all TODO comments