QmlDesigner.SubComponentManager: Do not show items without import

This feature is not working reliable, because we cannot add the import
and have proper metainfo for the item in the same transaction.

This is why we added the import manager.

Change-Id: Ifc6c34ae699f574989cfba8e07ec0d8c8d528101
Reviewed-by: Marco Bubke <marco.bubke@digia.com>