State: explicitly use internal state

[Garteal]

Explicitly uses the ImGuiGodot.Internal State instead.
Fixes name clashing if a project already has a State.

[pkdawson]

There shouldn't be a conflict unless there's a class called State in the global namespace. C# code should always be namespaced to avoid exactly this kind of problem.

Since C# 10 you can just stick namespace MyGameName; or whatever at the top of your .cs files. Godot will find your classes in any namespace.

[Garteal]

And that's exactly the setup I have right now. Of course it's better to namespace the whole thing, but IMO it should not be explicitly required. And IMO an external library/addon/whatever shouldn't cause this kind of name clash.