Allow main window to be controlled via dbus and the CLI

The persistent dbus service `org.gnome.Hamster.WindowServer' was removed in
favour of a dbus service created only whilst the hamster frontend is
running. This new service exposes a number of methods for easily displaying the
overview, preferences, statistics and about windows as well as the frontend's
main window, a feature that was sorely missing.

As before, it is possible to display any of the windows using the CLI.