Drop use of deprecated dbus.glib

When writing Python apps that will use asynchronous DBus calls and
signals, python-dbus must be initialised with the appropriate main
loop.

sugar currently does this by importing the datastore module from
sugar-toolkit-gtk3, and the datastore module uses the deprecated
and confusing dbus.glib module (which automatically and non-obviously
sets the default GLib main context as the DBus mainloop).

Eliminate the dbus.glib usage from sugar-toolkit-gtk3 as part
of the effort to make sugar initialised the DBus mainloop in a
more explicit and non-deprecated fashion.