compositor: only authorized client can bind desktop_shell
compositor: only authorized client can bind desktop_shell

Check, that only the desktop-shell client spawned by the compositor
(desktop-shell plugin) is allowed to bind to desktop_shell interface.
Other clients will receive an error like:

  wl_display@1.error(desktop_shell@20, 0,
  "permission to bind desktop_shell denied")

The error has the proper object id and interface type.

Note: desktop-shell cannot be started manually anymore, it has to be
started by the compositor automatically.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

krezovic/weston,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,Fantu/compositor-spice,eyolfson/weston,xorgy/weston,jonnylamb/weston,xorgy/weston,Fantu/compositor-spice,kwm81/weston,udoprog/weston,udoprog/weston,sir-murray/weston,Gnurou/weston,eyolfson/weston,mchalupa/weston,krezovic/weston,giucam/weston,Gnurou/weston,mchalupa/weston,kwm81/weston,jonnylamb/weston,giucam/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present