Visualization improvements

This issue is basically a continuation/iteration on https://github.com/ActivityWatch/aw-webui/issues/17

# Customizable views

- [x] Add ability for users to create/save/load user created views (done in #233)
- [x] Add ability to add/remove visualizations from the view (done in #233)
- [ ] Add ability to customize the size/width of a visualization?
- [x] Add ability to drag/drop visualizations while editing (done in #245)

# Timeline

- [ ] Allow for listing events in the combined timeline, similarly to ManicTime (#199)
- [ ] Allow for precision (start/end) time interval selection (currently only selectable by date range, not including time)
- [x] Make timeline full-width (no container)
- [ ] Group event timelines by host (partially depends on fixing hostnames for web/editor watchers) (#276)
- [ ] Make bucket titles shorter (to give more space for actual events)

# Trends

- [ ] Implement new 'Trends' view (#289)

# Sunburst

- [x] Add labels to sunburst arcs (https://github.com/David-Desmaisons/Vue.D3.sunburst/issues/14, https://github.com/ActivityWatch/aw-webui/pull/279)

# Calendar

- [ ] A calendar-like view with the full/combined timeline, grouped by app/category
- Might be more useful than the actual timeline
- FullCalendar looks good: https://fullcalendar.io/
- Previously discussed in https://github.com/ActivityWatch/aw-webui/issues/16

# Misc

- [x] Category colors (https://github.com/ActivityWatch/activitywatch/issues/416)