14: New UI & undo button

I have completely revised my UI elements for the displayed damage. I have created a simple blinking heart animation for the moment when some specimen attacks another one.

I also have introduced an undo button. After that, the player can undo all the moves he made after the last building option refill. This makes the game much more interesting as you don't need to plan everything very far ahead. You can always go back if something doesn't go as expected.