Option to check for entries that doesn't belong to any category

In the "batch operations" section, LR will pre-check entries that doesn't have any tags; it would be neat if this function get extended to categories as well, so that we can get to quickly find out which entries doesn't belong to any categories.

A situation where this could come in handy is for example when the user is manually archiving and adding many entries to LR (and listing them under various categories at the same time). In the event where the user might have missed/failed to add one or two entries to a category, such a function would allow them to discover and fix the issue much more conveniently.

Kinda related to #570.