* Enable custom themes to theme Compound
* Remove the now redundant username color variables
They are replaced by the Compound theming options (specifically, username colors can be themed by changing the color of Compound's decorative color tokens).
* Update our cancel icon
The cancel icon we're using in the app has drifted out of sync with the ones used in our designs. We also had two identical-looking icons, so this consolidates them into one.
I've simultaneously updated our chevron icons, since in the case of the 'jump to unread' timeline button, it became clear that the weight of the new close icon did not match the thinner chevron.
* Don't squish bottom/top-aligned tooltips near the edge of the screen
* Close the timeline panel when returning to the fullscreen timeline view
* Add layout switching capabilities to ElementCall
* Bring the room header in line with the group call designs
* Bring the PiP header in line with the group call designs
* Fix lints
* Clarify tooltip CSS calculations
* Test PipView
* Expand RoomHeader test coverage
* Test PipView more
