Bulk actions: show categories values and allow removal

[benoitchantre]

Currently, users can only use the bulk actions to assign categories to a selection of posts. They can't remove categories from the selected posts.

It would be a great improvement to show the states of the categories (terms in general) in the bulk actions:

• unselected
• indeterminate
• selected

Then to allow the user to unselect a category from the chosen posts.

There's an 8 years ticket in Trac for that.