This just allows the menu entry in Draw to reach
SfxMedium::SignContents_Impl(), and then call signDocumentContent(),
which shows the dialog (where the signature itself isn't visible yet).

Do this by not assuming a ZIP-based storage at various places when we're
not working with an "own" format.

Change-Id: I030d0f636d1aab98369024f3f10a975ea2786d59
Reviewed-on: https://gerrit.libreoffice.org/29774Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>