Convert FragmentHandler2::MCE_STATE to scoped enum and drop MCE_UNUSED
constant

Change-Id: Id0b3a81e61d77af5d3837527b008e196835f57cd
Reviewed-on: https://gerrit.libreoffice.org/33954Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>