Draft implementation of optional headerbars.
The only thing I noticed that is broken are missing widgets added to headerbars, for example search button in xfce4-settings-manager. I don't have any idea how to fix that from XfceTitledDialog's side, at least not in an elegant way.
Also notice that this change depends on Gtk/DialogsUseHeader and this setting is disabled by default, which I believe is not what we want, i.e. headerbars should be opt-out not opt-in. Alternatively a global setting under xsettings/Xfce could be introduced.