Dialog: Deprecate get_vbox(), replacing with get_content_area().

* gtk/src/dialog.hg: We forgot to rename get_vbox() to
get_content_area() for gtkmm 3.0, so we do it now via a
deprecation.
* demos/gtk-demo/example_change_display.cc:
* demos/gtk-demo/example_dialog.cc:
* demos/gtk-demo/example_sizegroup.cc:
* tests/dialog_deletethis/main.cc:
* tests/wrap_existing/main.cc: Adapted.