RE 7248_MantidEV_show_distance_between_last_points_picked

Initial version, works if Q is in sample coordinates for
both Q and HKL.  In order for this to work for HKL when
Q is in lab coordinates we will need to get and apply
the goniometer rotation.

refs #7248
Merge branch 'feature/7248_MantidEV_show_distance_between_last_points_picked' into develop