write COLOR function with extension namespace

and add to Excel import/export maps so it survives saving in that format

Change-Id: I3a8e07fdd1bded77d4bad60eefffeb0b1dbdd734
(cherry picked from commit 7c6a82d66ebe55fa5ce106bf60f841dd7758d2bc)
Reviewed-on: https://gerrit.libreoffice.org/11059
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>