Make layer-surface creation / destruction more self contained

Wanted to do s.th. completely different that was also touching layer surface code but this was bugging me for some time: When we started to move things so separate files the creation and destruction of layer surfaces got split between layer-surface and layer-shell (while the later should only do the protocol specific bits and leave all the listener setup, etc. to layer-surface). This series fixes this and also finally gets rid of the name vs prefix mismatch of phoc_layer_shell and layers.h and fixes a real issue as we forgot to disconnect a signal handler.

This brings layer surface a bit more in line with what we did for views (and their derived classes) a while back and also gets us way closer of making all the struct members in PhocLayerSurface private. By introducing yet another priv-> indirection we could make most of them private but let's rather avoid that and move all at once at some point.

Signed-off-by: Guido Günther agx@sigxcpu.org