Fixup Compare Drag-N-Drop

We have two new mime data types;

application/x-gc-intervals
application/x-gc-seasons

With packing routines into QMimeData declared
in IntervalTreeView and SeasonTreeView.

To get around an issue with overwrite mode on
drag and drop from the seasons / interval trees
we DO NOT ACCEPT the drop (even though we do
actually process the data).

All we need to do now is unpack the QMimeData in
ComparePane and then add the GUI elements to it etc.