[server] Add a subSurfaceTreeChanged signal to SurfaceInterface

The idea behind this signal is to notify whenever the tree of sub
surfaces changes in a way that a repaint of the Surface is required.
Possible situations are:
* surface damaged
* surface unmapped
* subsurface added/removed
* subsurface moved (position changed)
* subsurface stacking changed

Ideally it would be possible to provide the actual area which needs
repainting, but due to the possible complexity of the tree, synced
and desynced changes this doesn't look worth the effort. A user of
the signal might trigger too many repaints with it, but if it really
wants to be only notified about the actual changes, it can just track
the individual sub-surfaces.