Volume control

[mosra]

Next to start/stop commands a volume control could be usable. While it's redundant for DBUS players (which can be controlled easily by global shortcuts etc.), it is missing for Phonon.

There could be some indication in tray icon (like Amarok has for position in the track, but for volume) or in the menu/hover. Controlling volume with mouse wheel above tray icon should be an essential feature. Some players also spit out an notification while changing volume, so don't do it twice.

I don't know how it's about getting current volume level in various DBUS players, for some it is maybe set-only feature (with no visual feedback). It could be declared in AbstractPlayer and implemented in all subclasses:
enum Features {
VolumeControllable = 0x01,
VolumeGettable = 0x02
};
virtual int features() const = 0;