This adds a new context menu entry "Sign Signature Line" to signature lines.
You can directly sign the document from there.

Still missing is the updating of the graphic to reflect the valid/invalid
signature state.

Change-Id: Ib8bc8c57f9d5370a10e379f86f9e41e57c715e3f
Reviewed-on: https://gerrit.libreoffice.org/53778Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>