Magnets added, refactoring on blocks done.

Pickups and blocks behave differently. Pickups are a distance check.
Blocks are a trigger.
Player keeps track of all pickups. When a pickup comes into play,
it must be added to the player's list. Then removed by the pickup
object before destroy.
Bugfixes on magnet.

1. Distance check during pickup checks x and y (not just x)
2. Item purchase order fixed for green/red meter upgrades.