dexter relies on glib.get_user_data_dir() returning an existing dir

In dexter-rolodex/dexter/backend/models.py:32 from 0.18 it is assumed that USER_DIR (i.e. glib.get_user_data_dir()) exists. gio.make_directory() however does not create missing parents, so gio.make_directory_with_parents() should probably be used instead.

(This is related to https://bugzilla.redhat.com/show_bug.cgi?id=715964: In the build environment, the user data dir doesn't seem to exist, and because of DistUtilsExtra.auto, the (module level) code in models.py is run e.g. when calling "setup.py build".)