Allows to map an action to all devices.

This is accomplished by setting a special value (-1) to the device variable
in the InputEvent that's being used to compare with the one received from the OS.

This special value is invalid for a regular input, so it should be safe.
Implements #17942

core/input_map.h

@@ -39,6 +39,11 @@ class InputMap : public Object {
 	GDCLASS(InputMap, Object);
 
 public:
+	/**
+	* A special value used to signify that a given Action can be triggered by any device
+	*/
+	static int ALL_DEVICES;
+
 	struct Action {
 		int id;
 		List<Ref<InputEvent> > inputs;