Cleanup of c# api files and bindings generator

- We no longer generate RID and NodePath C# classes. Both will be maintained manually.
- We no longer generate C# declarations and runtime registration of internal calls for the following classes: RID, NodePath, String, GD, SignalAwaiter and Godot.Object (partial base).
- We no longer auto-generate the base members of Godot.Object. They will be maintained manually as a partial class.

This makes it easier to maintain these C# classes and their internal calls, as well as the bindings generator which no longer generates C# classes that don't derive from Godot Object, and it no longer generates the Godot.Object base members (which where unreadable in the bindings generator code).

- Added missing 'RID(Object from)' constructor to the RID C# class.
- Replaced MONO_GLUE_DISABLED constant macro with MONO_GLUE_ENABLED.
- Add sources in module/mono/glue even if glue is disabled, but surround glue files with ifdef MONO_GLUE_ENABLED.

modules/mono/csharp_script.cpp

@@ -107,7 +107,7 @@ void CSharpLanguage::init() {
 	gdmono = memnew(GDMono);
 	gdmono->initialize();
 
-#ifdef MONO_GLUE_DISABLED
+#ifndef MONO_GLUE_ENABLED
 	WARN_PRINT("This binary is built with `mono_glue=no` and cannot be used for scripting");
 #endif