[KPhotoAlbum] Super categories rework

[Tobias Leupold]

Hi list!

It would be very kind if someone reviewed my super category dialog rework I 
pushed to the "supercategories" branch. I think it's finished and working now, 
but I'm pretty sure there are some bugs to fix left.

This introduces a new concept for managing super categories.

A tree hierarchy is calculated and shown. The category combo box has been 
removed, as everything can be done via the tree view. Categories can be 
rearranged by dragging and dropping, as well as by selecting a category as a 
member of another one.

Additionally, I added the possibility to delete tags from this dialog, in the 
same way as this can be done from the annotation dialog (deleting super 
categories is of course also possible).

By the way, I found a bug and fixed it: empty groups were added to the 
database, but not read out after a restart (cf. commit 
02314e894bcbb19a99e6321a37bfe711212304ff).

If this goes to master, it will also fix Bug #237846.

Tobias