Add support for property groups

[BastiaanOlij]

Adding support for property groups and subgroups

Still need to add the correct methods to ClassDB for this, @vnen could use some pointers on how this should work :)

[vnen]

It's probably similar to how Godot itself does it: by adding a property with the PROPERTY_USAGE_GROUP usage, without any setter or getter.

[BastiaanOlij]

Got most of it in, just need to add classdb_register_extension_class_property_group to godot

[BastiaanOlij]

This now depends on godotengine/godot#52735

It seems to work but I'm still not getting properties for extensions shown in the inspector, I think that is a different issue however..

[vnen]

Thanks!