Fix custom assets being unmounted in scriptclass::hardreset()

This resulted in two bugs:
 1. Custom assets would not be unmounted when quitting to the menu.
 2. Custom assets would be unmounted when playtesting a level.

The solution is to unmount assets in Game::quittomenu() instead.

desktop_version/src/Game.cpp

@@ -7385,6 +7385,7 @@ void Game::quittomenu()
         createmenu(Menu::mainmenu);
     }
     script.hardreset();
+    FILESYSTEM_unmountassets();
 }
 
 void Game::returntolab()