Backport partially: CMS7-6250 Document workflow options disappear when not available: should be visible but inactive

Disabling workflow menu items instead of hiding them is needed for Advanced Search. However, we do not want to backport the UI changes for the normal workflow items (yet?), as that may be too intrusive for a minor release.

So: we only backport the option to disable a workflow menu item by calling setEnabled(false) on the workflow object.