use a native GtkAboutDialog on that platform and refactor the current cui about
dialog body to form the body of a vcl AboutDialog

use add_button to add the buttons to whichever is preferred of the headerbar
or action-area

Change-Id: I67e0b36dcb8d3fa08ec4f0397b0f6185b0778675
Reviewed-on: https://gerrit.libreoffice.org/73439
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>