st/widget: Switch to the new API to get actor from inputdevice
clutter_input_device_get_actor() was removed in favour of clutter_stage_get_device_actor(), so use that.
Counterpart of mutter!1633 (merged)
clutter_input_device_get_actor() was removed in favour of clutter_stage_get_device_actor(), so use that.
Counterpart of mutter!1633 (merged)