This allows to measure between features on a PCB. It uses a preview
EDA_ITEM in common, but due to the use of the IDs, it's currently
Pcbnew/Modedit only.
This also adds several "utils" for graphical functons useful when
drawing preview items on GAL canvases.
Fixes: lp:1467313
* https://bugs.launchpad.net/kicad/+bug/1467313
John Beard