[kwin_wayland] Split out wl_shell and wl_shell_surface into dedicated classes

New classes Shell and ShellSurface are created. Both are in shell.[h|cpp]
to indicate their close relationship with the Shell having to create the
ShellSurface.

WaylandBackend is adjusted to hold a Shell* and ShellSurface* instead of
the lower level structs. This also required adjustements to the creation
of the Backend as it now doesn't set a default size any more. Thus the
backendReady signal may not be emitted before the initial configure
event arrived. This also makes it easier to support either the fullscreen
shell or wl_shell at the same time.

Of course a unit test is added for the two new classes. This needs to
be extended once we have more control over the mock Wayland server.