Forms: don't show empty type fields
Sometimes types are created without subs. They are now hidden in forms to reduce clutter and because there is nothing to select anyway.
Additionally, warnings are displayed at the type overview in case a type has no subs or no associated classes because they won't be showed in forms.
Also this can be used to hide place types ("Administrative Units" and "Historical Places") in forms in case they are not needed in the project with simply removing all their subs.