Media controls redesign

The current MPRIS widget is a bit barebones and doesn't look very good.

Goals

• Gives access to the most common playback actions from the lockscreen or top drawer
• Shows the appropriate controls depending on the app (skip track for music, skip 30s for podcasts)
• Isn't in the way when there's nothing playing
• Looks good visually

Relevant Art

Android 11

• Media player is grouped with quick toggles
• Apps can do completely custom controls
• Two modes, compact (default) and expanded
• The latter can be very large and often includes lots of controls, including a full seekbar
• Multiple apps can have playback widgets at the same time, but they're stacked and swipeable horizontally

Experiment that didn't ship, IIRC:

older Android

Similar to 11, but

• The controls are at the top of the notification list instead of part of the quick toggles
• If there are multiple apps, the various players are above one another in the notification list

current iOS

• The widget is grouped with quick toggles in control center
• Only a single app can expose controls
• The widget is always there, even when nothing is playing
• Depending on the app there can be buttons to jump to the next track (music) or skip 30s (podcasts)

older iOS

macOS

• Only two buttons: pause/play and next track (is there a special mode for podcasts?)
• Does not go away when there's nothing playing

Windows 10x

Open Questions

• Can we show different buttons depending on the type of app?
• Do we want/need an actual progress bar?
• If so, we'd probably need an Android-style expanded mode. How would that work interaction-wise?

Related: #314, !664 (closed), !614 (closed)