NPC/spawner/level update

- added level: determines damage done and maximum health
- added more options to spawners (NPC-level/-attackrate/-attackrange)
- minor visual changes in the ingame-menu "SpawnerContol" registry,
which is now called "Cheats"
- enemy-AI now determines the distance to the player in a circle area,
instead of a square area
- player health is now displayed as a bar in the top left screencorner
- ingame-menu button has moved to the top right screencorner

Bugs:
- OnMouseDown() in EnemyAI doesn't work properly: it is only called with
a cetain value of NPCSpawner.activityRadius