Update and cleanup SnapAttract.

  Split the snap attract logic into three functions, one for Monitor,
  one for Windows, and one for Grid. Clean up SnapAttract code.

  Logic now 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.