Steam API, better bossfights and prettier mana fields

This update adds two major changes:

• The game now uses the Steam API. Currently I'm only using it to pause the game when the steam overlay is activated. But I'm planning to do more.
• Until now, many bosses only had one or two different attacks. To make the bossfights a bit more challenging I added a few more attacks.

And one minor change:

A few mana fields in the game still looked like in the early access version. I fixed that and now all mana fields have a coherent look.