Fixed up some compile issues

[BastiaanOlij]

Fixes bunch of issues that prevented me from compiling the nativescript-4 branch

[BastiaanOlij]

@vnen I changed some of the structs to classes because they were forward defined as classes with generate_bindings as well. Its not technically correct because I think they are still structs in Godot itself but it seems to work now without weird compiler and linker errors.

[vnen]

Thanks!