Minor fixes to "Measure distance" tool
Created by: ianrrees
Main change is to add a cursor that is activated along with the "Measure distance" tool, to give some visual feedback that the GUI expects the user to select points to measure between.
Also fixed (I think...) a small bug in the logic where a mouse button other than the primary one can be used to cancel out from the "Measure distance" tool, and edited some comments.