SvxChartColorTableItem has non-const members (aka modifiers)
which is bad for SfxItems in general, see comments in change.
Adapt SvxChartColorTableItem and it's usage in
SvxDefaultColorOptPage as needed.
This is also preparation for possible SfxItem refactoring

Change-Id: Ia7982b4e7bbfa736229223e55ce63e02143b8cf7
Reviewed-on: https://gerrit.libreoffice.org/69499
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@me.com>