This implements the subcompositor and subsurface protocol on both
Client and Server side.
Client:
New classes SubCompositor and SubSurface. The SubCompositor can be
created through the Registry and creates the SubSurface which is
bound to a Surface and has a parent Surface. The SubSurface class
provides convenient wrappers for all calls exposed in the
wl_subsurface interface.
Server:
New classes SubCompositorInterface and SubSurfaceInterface. Support
for all commands is added, though the API probably still can need
some fine tuning. The synchronized vs. desynchronized behavior is
not yet exposed in the API. This could also be delegated towards
the user of the library.
