manymanymany minor changes

-merged multiple window.clear() and window.display() calls from game &
mainmenu to application
-moved mousecursor into application, so its available in all program
states
-changed bgShapes random spawn area to windowsize
-added background texture for HUD (may need some improvement :P)
-improved camera behaviour (player is always in a defined rectangle)
-simplified continous shooting of player by using
sf::Mouse::isButtonPressed()