Preserve selection on selected items, require control key to unselect
Preserve selection on selected items, require control key to unselect

- Issue MatterHackers/MCCentral#3960
ListView selection behavior inconsistent with expectations

larsbrubaker/MatterControl,unlimitedbacon/MatterControl,jlewin/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,larsbrubaker/MatterControl,unlimitedbacon/MatterControl,jlewin/MatterControl,jlewin/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,jlewin/MatterControl