Started converting MainMenuSystem to a general menu system, as well as adding game over support
Made the error message a bit more descriptive
Preparing for game over
Removed some debug code I had left in by misstake
Game over picture
Added so the game can be restarted upon death
Removed debug code
Merge branch 'menusystem' into dev

Conflicts:
	se/chalmers/roguelike/Engine.java
	se/chalmers/roguelike/Systems/CombatSystem.java
	se/chalmers/roguelike/Systems/OverworldSystem.java