Further corrections to bed leveling wizard; improvements to priming code

Track active printing so that we only retract at end of print if we already primed at the start; parameterize leveling offset, and correctly incorporate in the offset in stored leveling height. Thanks to stevegt and Anders Olsson for tracking down the leveling repeatability issue (http://umforum.ultimaker.com/index.php?/topic/5935-z-axis-homing-inconsistent-on-um2-workaround-and-patch/?p=56588 et seq).