File Extension modifications to ExportLibraryItem

Modified code in ExportLibraryItemWindow to fix problems related to
saving files with extensions in Windows XP.
Merge remote-tracking branch 'Greg's_Fork/master' into development
Made the selected gcode layer match the first layer with rendering
made the window be focused when it is in gcode mode.
Merge branch 'development' of https://github.com/MatterHackers/MatterControl into development