Revert "Fix InputEvent device id clash" and add a compatibility function

This reverts commit 916d480686.

Revert "Fix InputEvent crash when opening project"
This reverts commit abb9c0f171.

Introduce a compatibility function for projects affected by the
device id clash reversal

Since the reverted PR introduced changes in the project.godot
file, it seems prudent to introduce a compatibility function for
such affected projects.

core/input/input_event.cpp

@@ -35,6 +35,9 @@
 #include "core/os/keyboard.h"
 #include "core/os/os.h"
 
+const int InputEvent::DEVICE_ID_EMULATION = -1;
+const int InputEvent::DEVICE_ID_INTERNAL = -2;
+
 void InputEvent::set_device(int p_device) {
 	device = p_device;
 	emit_changed();