Add a way to express an 'suspended' state

Applications may have ways to reduce resource consumption when not actively in use - for example when hidden behind other windows etc. One example here are browsers that throttle JS timers in such cases.

In the moment it is somewhat possible to do this by adding some heuristic around frame callbacks, however it might make sense to have an explicit hint, controlled by the compositor. For example for app overviews, where apps may not be woken up.

Some points from IRC:

• may or may not be defined as "visible"
• may or may not be defined as "watch your resources, noone cares about you right now"
• could be added to xdg-shell
• could be helpful on Xwayland/WMs as well to emulate similar X11 optimizations (XVisibilityEvent, map/unmap, NET_WM_STATE_HIDDEN)