PathListingWidget : Fix hang in Shift+click range selection
PathListingWidget : Fix hang in Shift+click range selection

This was only apparent in PathListingWidgets with more than one column, and occurred when we tried to build a range of paths between an index in one column and an index in another. In this case `QModelIndex::operator==` would never succeed because it includes the column in the comparison, and we would never exit the loop. We now compare just the internal pointers to the `PathModel::Items`, giving a column-insensitive comparison.

The change to `while( index0.isValid() )` is not necessary for this bugfix, but is a precaution against any similar hangs in the future. I'd rather get a "range selection not selecting the right things" bug report than a "range selection hangs" bug report.

johnhaddon/gaffer,GafferHQ/gaffer,hradec/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer