XP Magnet implemented

Applies a force to XP objects to pull them toward player
Increases radius with upgrades (use "4" hotkey to test)
Note that XP now has a drag applied to help make the XP magnet feel more
"magnet" like and prevent oscillations
See Constants.XP_DRAG, Constants.XP_GRAVITY, and
Constants.XP_MAGNET_FORCE_BASE to tweak the feel of the magnet/xp
falling.