[Window Manager] Sense when a window is marked urgent or demanding attention.

Post a notification and timer-activate windows demanding attention.
[Window Manager] 1) same urgency level for both kinds of signals. 2) Wait at least 10 seconds before setting focus to window demanding attention.
[Window Manager] Don't set focus to attention-seeking desktop window.
[Window Manager] Show a thumbnail along with the window icon in the notification.
[Window Manager] Destroy notification if alerting window is detected by timer function to have focus.