[DRAFT] GDExtension: Support hot-reloading in game

[dsnopek]

This adds a project setting (editor/run/enable_extension_reloading) that will enable hot-reloading GDExtensions in the game when run with the play button from the editor. (Currently, hot-reloading only happens in the editor itself, which is nice because you can see properties/signals/etc update, but doesn't let you "live edit" the game as it's running.)

It works in some really rudimentary testing on Linux.

However, I'm pretty sure this is going to have problems on Windows with the DLL copying stuff. I'd like to do more testing, including on multiple platforms, before taking this out of draft.

[dsnopek]

Thinking about this some more, this might make more sense (from a UI perspective) as a checkbox in the "Debug" menu, like "Synchronize Scene Changes" and "Synchronize Script Changes". I'm not sure if it'd fit in from a technical perspective, though.

[Repiteo]

This should fix the unused-variable errors

Suggested change

	bool gdextension_reload_in_game = false;
	#ifdef TOOLS_ENABLED
	bool gdextension_reload_in_game = false;
	#endif // TOOLS_ENABLED

[dandersch]

@dsnopek Thanks for implementing this so quickly.

I can't confirm that this is working in a basic project on Linux.

I'm using the godot-cpp bindings on branch 4.3 for the extension and I run the editor with --gdextension-reload and with "Enable Extension Reloading" turned on (running with only --gdextension-reload doesn't seem to do anything). When running the game with the play button and then recompiling the extension with a change, after focusing the editor, the change is reflected inside the editor, but the changed node just disappears inside the game window (the node being just a moving godot icon sprite).

I also have the problem that the editor only ever hot-reloads the extension when I refocus the editor, i.e. not while I stay actively on it. That's probably unrelated to this commit, I just wonder if it's like this for others.