Unstable sorting should be slightly faster than stable sorting, and if
the vector was built asynchronously then preserving the initial order
doesn't matter too much.
Continue to use stable sorting where the vector is not built
asynchronously, or if the vector is partially sorted (e.g. when new
elements are pushed to a sorted vector).
The panel buttons for next/previous didn't do anything since `on_press`
wasn't set. Now they work.
Fixed `autosize` which I had accidentally committed after using it for
testing. (It would be good to figure out why the applets don't work
properly with autosize when run outside of the panel, since that's handy
for testing.)
I noticed with VLC, skipping a track makes the media controls disappear
very briefly, and sometimes the pause button didn't return. I guess
`can_play` (etc.) are temporarily set to false, probably expecting the
controls to be greyed out and not hidden from a panel. Anyway,
monitoring for changes to these properties as well will hopefully fix
the part where it could remain missing. Though I can't reproduce that
consistently.
