More arms + UI improvements

Polished most interfaces. Nothing too big, but they should look a little nicer now.

Also finally implemented arm mix-and-matching for enemies. At the start of the battle, the enemies will equip themselves with two arms at random from a selection of possible arms. And this isn't just a cosmetic change. Different arms will grant the enemies different abilities and behaviors.

For instance, an enemy with a blade and a shield will use standard attacks more often and have more health, while an enemy with a grapnel and a buzzsaw will force you to start the battle with one less card in your hand and will occasionally attack your deck to remove cards from it before you get a chance to use them.

As of now there are 8 different types of arms implemented. If my math doesn't fail me, that translates into 36 different possible combinations already. And I'm considering giving the enemies additional arms at higher difficulty levels, which would make the number of combinations skyrocket.