Variant: Rename Type::_RID to Type::RID

The underscore prefix was used to avoid the conflict between the `RID` class
name and the matching enum value in `Variant::Type`.

This can be fixed differently by prefixing uses of the `RID` class in `Variant`
with the scope resolution operator, as done already for `AABB`.

editor/editor_properties_array_dict.cpp

@@ -892,7 +892,7 @@ void EditorPropertyDictionary::update_property() {
 					prop = memnew(EditorPropertyNodePath);
 
 				} break;
-				case Variant::_RID: {
+				case Variant::RID: {
 					prop = memnew(EditorPropertyRID);
 
 				} break;