For signing, preselect the key in the list of available keys.
For encryption, when 'encrypt to self' bit is set, add user's key
to the list of keys to encryption.

Change-Id: I5bbfd0e2cc97b76c1304a2a345a51cf83bc5949e
Reviewed-on: https://gerrit.libreoffice.org/46694Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>