Expose DEVICE_ID_EMULATION constant in InputEvent

This also improves the documentation related to emulated InputEvents.

core/input/input_event.cpp

@@ -132,6 +132,8 @@ void InputEvent::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("xformed_by", "xform", "local_ofs"), &InputEvent::xformed_by, DEFVAL(Vector2()));
 
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "device"), "set_device", "get_device");
+
+	BIND_CONSTANT(DEVICE_ID_EMULATION);
 }
 
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