XfwWorkspaceGroupDummy errors

With latest xfdesktop build using libxfce4windowing, in addition to the build patch that just went through, workspaces don't work:

(xfdesktop:366516): GLib-GObject-CRITICAL **: 16:41:02.820: ../glib/gobject/gsignal.c:2625: signal 'workspace-created' is invalid for instance '0x5e9d9fd35a10' of type 'XfwWorkspaceGroupDummy'

(xfdesktop:366516): GLib-GObject-CRITICAL **: 16:41:02.820: ../glib/gobject/gsignal.c:2625: signal 'workspace-destroyed' is invalid for instance '0x5e9d9fd35a10' of type 'XfwWorkspaceGroupDummy'

We need to also migrate the callbacks. Incoming merge request...