filter: don't loose signing description during pdf export

The free-form string attached to a signature is called description
during ODF/OOXML signing. The certificate chooser dialog has an input
field to provide that.  The PDF export dialog's signature tab reuses
this dialog, but also provides an an own reason input field for the same
purpose.

So in case the generic dialog's description field is filled, don't
simply throw away that string, but set the pdf export's reason field to
the same value.

XDocumentDigitalSignatures.idl is not a published interface and it is
used only internally, so the API change is only nominal.

Change-Id: I6d4cf0b3f586417a76a052dc30c960478a95c984
diff --git a/filter/source/pdf/impdialog.cxx b/filter/source/pdf/impdialog.cxx
index a4efda0..a2ff5f7 100644
--- a/filter/source/pdf/impdialog.cxx
+++ b/filter/source/pdf/impdialog.cxx
@@ -1758,7 +1758,9 @@
        security::DocumentDigitalSignatures::createWithVersion(
            comphelper::getProcessComponentContext(), "1.2" ) );

    maSignCertificate = xSigner->chooseCertificate();
    // The use may provide a description while choosing a certificate.
    OUString aDescription;
    maSignCertificate = xSigner->chooseCertificate(aDescription);

    if (maSignCertificate.is())
    {
@@ -1768,6 +1770,7 @@
        mpEdSignPassword->Enable();
        mpEdSignContactInfo->Enable();
        mpEdSignReason->Enable();
        mpEdSignReason->SetText(aDescription);

        try
        {
diff --git a/offapi/com/sun/star/security/XDocumentDigitalSignatures.idl b/offapi/com/sun/star/security/XDocumentDigitalSignatures.idl
index cc785b9..5ee5b9e 100644
--- a/offapi/com/sun/star/security/XDocumentDigitalSignatures.idl
+++ b/offapi/com/sun/star/security/XDocumentDigitalSignatures.idl
@@ -131,8 +131,10 @@
    void    addLocationToTrustedSources( [in] string Location );

    /**  This method shows CertificateChooser dialog, used by document and PDF signing

         @since LibreOffice 5.3
     */
    com::sun::star::security::XCertificate chooseCertificate();
    com::sun::star::security::XCertificate chooseCertificate( [out] string Description );

} ;

diff --git a/xmlsecurity/source/component/documentdigitalsignatures.cxx b/xmlsecurity/source/component/documentdigitalsignatures.cxx
index eb39cafa..cfa3d24 100644
--- a/xmlsecurity/source/component/documentdigitalsignatures.cxx
+++ b/xmlsecurity/source/component/documentdigitalsignatures.cxx
@@ -440,7 +440,7 @@
    return bFound;
}

Reference< css::security::XCertificate > DocumentDigitalSignatures::chooseCertificate() throw (RuntimeException, std::exception)
Reference< css::security::XCertificate > DocumentDigitalSignatures::chooseCertificate(OUString& rDescription) throw (RuntimeException, std::exception)
{
    Reference< css::xml::crypto::XSecurityEnvironment > xSecEnv;

@@ -454,6 +454,7 @@
        return Reference< css::security::XCertificate >(nullptr);

    Reference< css::security::XCertificate > xCert = aChooser->GetSelectedCertificate();
    rDescription = aChooser->GetDescription();

    if ( !xCert.is() )
        return Reference< css::security::XCertificate >(nullptr);
diff --git a/xmlsecurity/source/component/documentdigitalsignatures.hxx b/xmlsecurity/source/component/documentdigitalsignatures.hxx
index 7ab6405..bd07304 100644
--- a/xmlsecurity/source/component/documentdigitalsignatures.hxx
+++ b/xmlsecurity/source/component/documentdigitalsignatures.hxx
@@ -96,7 +96,7 @@
    void SAL_CALL addAuthorToTrustedSources( const css::uno::Reference< css::security::XCertificate >& Author ) throw (css::uno::RuntimeException, std::exception) override;
    void SAL_CALL addLocationToTrustedSources( const OUString& Location ) throw (css::uno::RuntimeException, std::exception) override;

    css::uno::Reference< css::security::XCertificate > SAL_CALL chooseCertificate( ) throw (css::uno::RuntimeException, std::exception) override;
    css::uno::Reference< css::security::XCertificate > SAL_CALL chooseCertificate(OUString& rDescription) throw (css::uno::RuntimeException, std::exception) override;
};

css::uno::Reference< css::uno::XInterface > SAL_CALL DocumentDigitalSignatures_CreateInstance(