Make it more clear how to add a taxonomy category

To edit taxonomy terms for a document you can use the taxomy picker. Select terms from the tree, or remove items already selected. Even after working for a week with this picker I ever so often forget to click on the "add category" link after selecting a new category.

This can be simply improved with changing the link into a button, which is much more visible than a link. It would also help a lot if the OK button on the picker would only be enabled if a change to the selection was made (add or remove a key).