ShowMessageBox if users try t to add new printer while a print is either active or paused
ShowMessageBox if users try t to add new printer while a print is either active or paused

MatterHackers/MatterControl#1299

mmoening/MatterControl,larsbrubaker/MatterControl,jlewin/MatterControl,mmoening/MatterControl,tellingmachine/MatterControl,MatterHackers/MatterControl,larsbrubaker/MatterControl,larsbrubaker/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,rytz/MatterControl,jlewin/MatterControl,MatterHackers/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,rytz/MatterControl,unlimitedbacon/MatterControl,tellingmachine/MatterControl,rytz/MatterControl,unlimitedbacon/MatterControl,MatterHackers/MatterControl,jlewin/MatterControl,jlewin/MatterControl