[Shotwell] Duplicate tag created if entered while in edit mode

[Dougie Nisbet]

I've checked to see if there's a bug already open for this so apologies 
if I've missed it.

I've discovered that if I add an already existing tag to an image while 
in Edit Mode the tag is added as a duplicate as a top level tag. This 
doesn't happen if in browse (thumbnail) view.

Dougie