GtkDialog: rejigger headerbar handling

GtkDialog: rejigger headerbar handling

We can't avoid calling set_titlebar in buildable_add_child, since
GtkBuilder assumes that it can e.g. call grab_default at the end,
which only works if the widget is inside a toplevel at that point.

Instead, drop the titlebar and clean up priv->headerbar in constructed
if we find that we don't want a headerbar.