This method shows the certificate choosing dialog previously used in ODF
signing and returns the selected certificate. For PDF signing support,
we need to reuse existing CertifacateChooser class of xmlsecurity module.
So the UNO api of ::com::sun::star::security::XDocumentDigitalSignatures is
extended.

Change-Id: I09f7d0ee180b16b588798820206780df359d133e