Resolve WB-577: Crash on checking 'On' box for Parcel Reordering under the...

Resolve WB-577: Crash on checking 'On' box for Parcel Reordering under the 'Charting' tab in Overlay Toolbox:  The checkbox was enabled even if there were no parcel label files.  The checkbox is now enabled only if there is a parcel label file available for selection.  This prevents the crash.