Adds 'exposed' field to ClassInfo

This field represents if the class is exposed to the scripting API.
The value is 'true' if the class was registered manually ('ClassDB::register_*class()'), otherwise it's false (registered on '_post_initialize').
- Added missing registration of classes that are meant to be exposed.

main/main.cpp

@@ -256,6 +256,7 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 
 	translation_server = memnew(TranslationServer);
 	performance = memnew(Performance);
+	ClassDB::register_class<Performance>();
 	globals->add_singleton(ProjectSettings::Singleton("Performance", performance));
 
 	GLOBAL_DEF("debug/settings/crash_handler/message", String("Please include this when reporting the bug on https://github.com/godotengine/godot/issues"));