xdg-shell: add urgency hint

Two new requests are added to xdg_toplevel: set_urgent and unset_urgent.
They allow clients to set and unset an urgency hint on their toplevel.
The urgency hint can be ignored by the compositor.

This is similar to ICCCM's UrgencyHint in the X11 world. It can be used
e.g. by messaging applications when receiving a new message or by
terminal emulators when a command completes.

Signed-off-by: Simon Ser <contact@emersion.fr>
2 jobs for urgency in 1 minute and 4 seconds (queued for 12 seconds)