Improve & fix Mono build

- Fix C++ compile errors about pending variable renames after the `Reference` to `RefCount` change.
- Fix C# compile errors due to the recent rename of `EnablePlugin()` and `Build()`, which are now underscore-prefixed in bindings.
- Additional rename: `godot_icall_Reference_Dtor` to  `godot_icall_RefCounted_Dtor`.

modules/mono/mono_gd/gd_mono_utils.cpp

@@ -114,7 +114,7 @@ MonoObject *unmanaged_get_managed(Object *unmanaged) {
 		// Unsafe refcount increment. The managed instance also counts as a reference.
 		// This way if the unmanaged world has no references to our owner
 		// but the managed instance is alive, the refcount will be 1 instead of 0.
-		// See: godot_icall_Reference_Dtor(MonoObject *p_obj, Object *p_ptr)
+		// See: godot_icall_RefCounted_Dtor(MonoObject *p_obj, Object *p_ptr)
 		rc->reference();
 		CSharpLanguage::get_singleton()->post_unsafe_reference(rc);
 	}