application: Add dbus register/unregister hooks

application: Add dbus register/unregister hooks

When the application is using its D-Bus backend, it is useful to be able
to export extra D-Bus objects at the right time, i.e. *before* the application
tries to own the bus name. This is accomplished here by adding a hook
in GApplicationClass for this; and a corresponding hook that will be called
on unregistration to undo whatever the register hook did.

Bug #675509.