Add a tab to the launcher for handling datafolders

There is the great feature of having/using more than one DataFolder at the same time in OpenMW. It would be nice if DFs could be handled (add/remove/activate/deactivate) with the launcher as it is done with the esp files. Add/remove/activate/deactivate of BSA files could be done in the launcher too.

(RM-2858 from redmine: created on 2015-08-15 by Who Knows, , relates #2857 (closed))