fdo#80161 Add an icon to the 'Export as PDF' command

Change-Id: I09972ebec71a332d2f5a8630df7b5fecd2b78c87
Reviewed-on: https://gerrit.libreoffice.org/12228
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>