Drop dependency on PyXDG, use GLib instead

There's really no reason to depend on an external Python library when
GLib has a perfectly good implementation already exposed via
pygobject.

Specifically I don't want to pull in pyxdg to gnome-continuous.

https://bugzilla.gnome.org/show_bug.cgi?id=711184
Drop use of old "glib" module

It's not shipped in gnome-continuous, and we were just using
it for markup_escape_text which works fine via gi.repository.GLib.

https://bugzilla.gnome.org/show_bug.cgi?id=711184