Improvements to UI and balancing

-Fix barrier enemies appearing with swords instead of pushing barriers
-Change position of ability buttons to be under the castle
-Change  the ability unlock icon to show how many kills are required for unlock
-Change the required number of kills for ability unlocks (from 100, 200, and 300 to 50, 100, and 150)
-Change name of rapidfire to autofire
-Add info text boxes when hovering over abilities to explain what they do.
-Add 'Unlocked' animation when an ability is unlocked to try to draw the players attention to the ability