Implement pause-aware picking

This changes the way 2D & 3D physics picking behaves in relation to pause:
- When pause is set, every collision object that is hovered or captured (3D only) is released from that condition, getting the relevant mouse-exit callback., unless its pause mode makes it immune from pause.
- During the pause. picking only considers collision objects immune from pause, sending input events and enter/exit callbacks to them as expected.
- When pause is left, nothing happens. This is a big difference with the classic behavior, which at this point would process all the input events that have been queued against the current state of the 2D/3D world (in other words, checking them against the current position of the objects instead of those at the time of the events).

scene/main/scene_tree.cpp

@@ -409,6 +409,7 @@ bool SceneTree::physics_process(float p_time) {
 	emit_signal("physics_frame");
 
 	_notify_group_pause("physics_process_internal", Node::NOTIFICATION_INTERNAL_PHYSICS_PROCESS);
+	call_group_flags(GROUP_CALL_REALTIME, "_viewports", "_process_picking");
 	_notify_group_pause("physics_process", Node::NOTIFICATION_PHYSICS_PROCESS);
 	_flush_ugc();
 	MessageQueue::get_singleton()->flush(); //small little hack