Code review comment for lp:~straemer/software-properties/fix-for-1058070

The improvement looks fine, and it makes software properties a "singleton" app, allowing it to run only one instance per session.

However it does the single-instance handling via a custom dbus interface. Gtk Application [1] can do single-instance handling since gtk 3.6, along with other maybe-useful features, like appmenu, etc. I don't know whether there are any best-practices regarding single-instance apps, but if it has been added to the core gtk platform, maybe we could make use of that (I guess that GObject introspection works with the gtkapplication class too, so it shouldn't be too tricky)

[1] http://developer.gnome.org/gtk3/3.6/GtkApplication.html