shell: automatic exec and kill of wscreensaver
shell: automatic exec and kill of wscreensaver

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

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