shell: Add fullscreen path into activate()

shell: Add fullscreen path into activate()

Use shell_stack_fullscreen() to raise fullscreen surface in activate(),
and before activate() a regular surface, move all the fullscreen
surfaces from fullscreen_layer to toplevel_layer.

Also add a void *private into struct wesont_surface since we already
have a configure() vfunc. That helps to get the associated fullscreen
surface of black surface.