At the request of bug tdf#94957, adding unicode conversion
shortcut (alt+x) to Math also.

Change-Id: I5b4d17543a819f035ade0d9a516eeef8542a286e
Reviewed-on: https://gerrit.libreoffice.org/19323Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>