Long waiting times

[suluke]

Especially with the new back button implementation, it is becoming pretty annoying that the LevelsOverviewScreen is not being cached. A good place to do this would be the package controller (at least just caching the package, if not the screen).

While investigating this, it would also be nice to reduce the code necessary to set the last LevelsOverviewScreen. Currently it is

game.showLevelOverviewScreen(game
                .getLevelPackagesController()
                .getLevelController(gameController.getLevel().getPackageIndex()));

In addition, it would be easy to change GameController.createPlacementScreen into a getPlacementScreen to cache this sort of screen. Its creation also causes unnecessary overhead and thus waitng time