Update SnapAttract Logic.

  Split the snap attract logic into three functions, one for Monitor,
  one for Windows, and one for Grid. This is to allow calling the functions
  in different orders to control which object takes priority when determining
  what to snap to.

  First checks for Monitor edges and window edges and snaps to closest.
  If no monitor or window edge found, snap to SnapGrid.

  Update EdgeMoveResistance to use the same function for Monitor snapping.

  Fixes #631.