Improvements to adding digital signatures

 * Don't make WidgetAnnotation know about signatures stuff, widget
   annotations are for multiple things
 * Don't create an "empty" widget annotation and then call sign on it
   (which is wrong because widget annotations can be multiple things),
   just say sign the document with this data (cert, l&f, etc)
 * Remove the "management" functionality from CertificateTools it was
   only visual, i.e. it didn't really add/remove certificates
 * Ask for the NSS password (if needed)