Fixes https://github.com/pop-os/cosmic-applets/issues/335.
This calculates volume as a percentage of `Volume::NORMAL`, which seems
to be the correct way to display PulseAudio volumes to the user. This
matches `cosmic-osd`, and seems to match alsamixer.
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.
