This will always show 1 unit from each system, with the primary unit
being the current frame unit and the secondary unit being the
most recent unit used from the other system. These are saved in
the settings, so they are saved between runs.
SELECTION now holds EDA_ITEMs not BOARD_ITEMs so various places had to
change to casting the selected items to BOARD_ITEMs.
Fixed compilation warnings on clang (Tom)
Jeff Young
Ian McInerney
Jon Evans