clutter: Store device->actor associations in ClutterStage
With the introduction of the input thread, we want to avoid modifying ClutterInputDevices from the main thread, since they're owned and updated by the thread.
There's one part of ClutterInputDevice that's still modified from the main thread though, that is device-actors of pointer devices, this MR moves that state-tracking into ClutterStage instead.
Shell counterpart is gnome-shell!1537 (merged)
Edited by Jonas Dreßler