Fix the lack of unregistering interval functions at screen destruction

im fairly sure this is the right thing to do
not doing this seemed to let creating and deleting interval functions then restarting screens do very very very weird things in lua (such as hardcoded strings being referred to as arbitrary classes such as "315" and "ActorFrame" thus breaking very tiny things that you didnt notice until it crashed the game)
quite a funny consequence

src/Etterna/Screen/Others/Screen.cpp

@@ -35,7 +35,19 @@ Screen::InitScreen(Screen* pScreen)
 	pScreen->Init();
 }
 
-Screen::~Screen() = default;
+Screen::~Screen()
+{
+	// unregister the periodic functions
+	auto* L = LUA->Get();
+	for (auto& dfunc : delayedPeriodicFunctions) {
+		auto id = std::get<3>(dfunc);
+		luaL_unref(L, LUA_REGISTRYINDEX, id);
+	}
+	delayedFunctions.clear();
+	delayedPeriodicFunctions.clear();
+	delayedPeriodicFunctionIdsToDelete.clear();
+	LUA->Release(L);
+};
 
 bool
 Screen::SortMessagesByDelayRemaining(const Screen::QueuedScreenMessage& m1,