Create simple UI for Abilities

Each ability equipped by the player should have a simple icon indicating the following:
* ~~Ability has an icon to indicate which ability is equipped~~
* ~~Ability has a 'cooldown state' when it cannot be used~~
* ~~Ability has a hotkey to show what key to press to use it~~

Bonus bugfix: Accidentally stuck a bugfix for the player walking through walls in this branch too :)