convert vcl StateChangedType to enum class

Change-Id: Ifa46523619ae7fb8d112525b8df594836e51bb8f
Reviewed-on: https://gerrit.libreoffice.org/11843
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>