GimpData: add gimp_data_set_folder_tags() and remove the tag logic
from gimp_data_set_filename(). The function gets the toplevel data
directory passed so it knows where to stop assigning tags.

GimpDataFactory: when loading data, keep track of the currently
processed data hierachy's toplevel directory, and pass it to above new
function. Also make sure obsolete files don't get folder-tagged.