Add simple volume control to Media Controller applet. Volume can by changed by:
- mouse wheel on applet icon (3 % step), and
- new global shortcuts (5 % step, unbound by default).
There is no mute support currently (mpris interface does not have a mute method so we would have to keep
track of old volume).
Currently the patch does only prevent volume from being negative but it will try to set volume larger than 1 (mpris spec
does not prevent players from using larger volumes).
BUG: 386588