DisplayManager: ignore power key events when suspended

Signed-off-by: Simon Busch <morphis@gravedo.de>
DisplayManager: when we get a power key press event while suspended defer it

Signed-off-by: Simon Busch <morphis@gravedo.de>
BootManager: expose compositor availability

Signed-off-by: Simon Busch <morphis@gravedo.de>
DisplayManager: respect boot status when switch states

Signed-off-by: Simon Busch <morphis@gravedo.de>
BootManager: activate/deactivate suspend according to the compositor state

Signed-off-by: Simon Busch <morphis@gravedo.de>