Add volume slider back

Tried to make it more reliable, but not sure if all weird cases covered.

Loosely based on 97ecb434, but also made following changes:

• Now before playing it always checks current volume value
• Slider moved to the dedicated popover with button showing icon corresponding to current volume level (this can be changed if you have different vision where it should be)
• Also slider now presents volume as qubic values instead of linear (see this)
• now support MPRIS volume manipulation (but I couldn't figure out how to make it reflect changes to volume made outside of MPRIS)

Also I'm sure there are some bugs left that I didn't notice.