Varied changes

Updated to almost latest version of libGDX
Changed backend to jglfw instead of lwjgl (mac reasons)
Tile.use override now optional and implements common functionality in
the base class
Tick rate changed from 60 to 50 and delay times reduced for it
Added a box abstract item class, and "THE BOX" which is an in-joke
Added running shoes (increases speed)
Fixed minor bug in sliders that probably didn't get triggered anyways
Added convenience function to add items to player
FPS now drastically reduced when game is in background (bug: when
starting, game thinks it's in background)