Mutually exclusive categories in editor

[fxedel]

Since #913, the fixture test has mutually exclusive category groups. One could extract these groups to a shared resource (maybe directly in the schema?) and then use them in the fixture editor: If one category of such a group is used, e.g. Scanner, the other categories (Barrel Scanner and Moving Head) are disabled. Maybe add a hint like "Category 'Moving Head' can't be used along with 'Scanner'" then.