Dont crash when surfaces and clients disappear
Dont crash when surfaces and clients disappear

Set up a notification system, so we get a callback when a client and its
surfaces disappear and can drop references the lost surface.

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