Removes drop indication for tag reordering
Removes drop indication for tag reordering

- closes #65

When tag is dragged, tree no longer indicates that it is possible to
reorder tags by dropping them on different possition in same level of
hierarchy. Tags can only be reparented using drag/drop functionality.

mono/f-spot,mans0954/f-spot,dkoeb/f-spot,mans0954/f-spot,dkoeb/f-spot,dkoeb/f-spot,NguyenMatthieu/f-spot,dkoeb/f-spot,mono/f-spot,NguyenMatthieu/f-spot,dkoeb/f-spot,dkoeb/f-spot,mono/f-spot,mans0954/f-spot,mono/f-spot,NguyenMatthieu/f-spot,mans0954/f-spot,mono/f-spot,mans0954/f-spot,NguyenMatthieu/f-spot,mono/f-spot,mans0954/f-spot,NguyenMatthieu/f-spot