Implemented basic Tooltip UI

It's really ghetto.
The String command in each button needs to be replaced with a
BuildingType from the BuildingType enum. That whole system is pretty
rough. This is my first time doing this, after all!
Next up: Improving this UI (eg. text wrapping), implementing other UI
(eg, gameover screen, resource amounts)
Improving the enemy base building AI (not cheating...)
Improving the unit attack AI (attack other buildings between the unit
and HQ)