No need to synchronize twice

One time is enough.
A single overlay for every screen

Overlays are windows without input, one for every screen.
The actual contents are changed as needed, for now we only support the
volume control.

Issue: #43
Mute master volume

Implement getter and setter for mute.

Issue: #55
Emit a signal when volume is increased or decreased

Emit a signal right after we increase or decrease volume.

Issue: #55
Rename volume control slots

No functional change.
Correctly set master volume

Set volume to front left and front right.

Issue: #55
Volume overlay

Finish the volume overlay.

Issue: #55