Move LoadingScreen to a global.

LoadingScreen is a variable that should be available to all loading
definitions.  Param does this currently, but I'm redesigning that, and this
particular case doesn't fit.  Conceptually, it really is a global (though
perhaps should be tucked away with an environment or something).

stepmania/src/Screen.cpp

@@ -18,9 +18,10 @@
 void Screen::InitScreen( Screen *pScreen )
 {
 	/* Set the name of the loading screen. */
-	ActorUtil::ActorParam LoadingScreen( "LoadingScreen", pScreen->GetName() );
+	LUA->SetGlobal( "LoadingScreen", pScreen->GetName() );
 
 	pScreen->Init();
+	LUA->UnsetGlobal( "LoadingScreen" );
 }
 
 Screen::~Screen()