Added ability to override shortcut definitions for OSX (remove preprocessor defines)

[EricEzaM]

May help with godotengine/godot-docs#3333 cc @pycbouh

No more OSX_ENABLED preprocessor define checks when defining default editor shortcuts. Just add a call to ED_SHORTCUT_OVERRIDE with the corresponding feature you want the override to be for (e.g. "macOS"), and the new event.

[EricEzaM]

@akien-mga Ok, I changed it. I think it is right but it will need testing on macOS of course.

[YuriSizov]

So, to me changes look sane and clear, but I wouldn't dare to approve this without testing it on a Mac first, which I unfortunately cannot do 🙃

Having to supply shortcut names to the overrides may be a bit excessive but we can always address this in a separate PR without breaking anything.

[EricEzaM]

Ok, my previous implementation was actually wrong and would not work properly, lol. I think I was rushing my thinking a bit.

I have tested the new version on Windows, by adding a shortcut override with "Windows" as the feature. This worked correctly:

	ED_SHORTCUT("editor/fullscreen_mode", TTR("Toggle Fullscreen"), KEY_MASK_SHIFT | KEY_F11);
	ED_SHORTCUT_OVERRIDE("editor/fullscreen_mode", "macOS", KEY_MASK_CMD | KEY_MASK_CTRL | KEY_F);
	ED_SHORTCUT_OVERRIDE("editor/fullscreen_mode", "Windows", KEY_MASK_CTRL | KEY_KP_1);

The shortcut was correctly listed as Ctrl + KP1 in editor settings, and the shortcut worked when pressed.

@pycbouh Additionally with this, the shortcut name is no longer repeated. It should now also be much easier to parse the codebase with a Python file (or similar), looking for ED_SHORTCUT_OVERRIDE to automatically construct the list of editor shortcuts.

[EricEzaM]

Ok, I think it is in a mergeable state now. I have not tested on macos, but really it should not matter, it all works on the Feature system now. So doing either of the following should not matter:

// macos
ED_SHORTCUT_OVERRIDE("editor/fullscreen_mode", "macos", KEY_MASK_CTRL | KEY_KP_1);
// windows
ED_SHORTCUT_OVERRIDE("editor/fullscreen_mode", "windows", KEY_MASK_CTRL | KEY_KP_1);

Tested on windows and it works fine. If you are on Linux you can pull this PR and add an ED_SHORTCUT_OVERRIDE somewhere with the feature being linuxbsd and it should work just as well.

[akien-mga]

Thanks!