ComparePane: setDropAction on drop event

.. so it can be accepted and the data is not wiped
   or removed in the source treewidget.
Drag onto athlete tab to switch tabs

.. this allows us to drag and drop between athletes or
   to switch athlete if dropping from outside of GC.

.. also added context to the mime data so the compare pane
   will know where the dragged object has come from (ie. it
   may be from a different athlete).