fdo#75256 Improve Sifr Icon Theme

New insertauthorfield, underlinedouble, upperline and updated shadowed icons.
The a of the shadowed icon looked thinner and is looking like the other format icons now.

Change-Id: I3bd7e68df5e4a2686be022605a96dd92eb86a200
Reviewed-on: https://gerrit.libreoffice.org/12183
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>