gnome-shell-calendar-server: Initialize GTK+

If evolution-data-server needs to prompt for a password, it will try
to pop up a GTK+ dialog. When GTK+ is not initialized, the result is
a crash. So, initialize GTK+ and run a main loop.

See https://bugzilla.redhat.com/show_bug.cgi?id=809681

The result is ugly since we have a Gnome-shell-calendar-server fallback
application, but I don't think it's worth installing a desktop file
and having a string break, since this is pretty uncommon (only for
manually added calendars without the password stored in gnome-keyring),
and apparently this is being rewritten for 3.5 to have the dialogs come
the e-d-s daemon rather than from the individual application.

https://bugzilla.gnome.org/show_bug.cgi?id=673608