new statusbar element property mandatory=true/false to determine
if this element can be hidden if total statusbar width is not
enough to fit all elements.

marked some calc and draw statusbar elements as not mandatory.

Change-Id: I20e26d3c4bd865e94ea48632a1e97d55f3fa712f
Reviewed-on: https://gerrit.libreoffice.org/56443
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>