Turn SfxItemState into a C++11 scoped enumeration

...to gain further confidence in the claim "that none of the existing
code tries to uses combinations of these enum values"
(d92602c5b13d0a60439d86c5a033d124178726ca "more fixes for SfxItemState")

Change-Id: I987922d945e8738e38adfde83b869adf3ff35b13
Reviewed-on: https://gerrit.libreoffice.org/11384
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>