Taxonomy: improve usability by adding values instantly on select

To add rtaxonomy values to documents with the taxonomy picker dialog, one has to first select a value in the tree and then use the "Add category link".

It would save one click per selected item (and moving the mouse back and forth) if the picker can be configured to add values instantly when they are selected in the tree.

A different approach would be to have checkboxes before every tree item, but I don't know if that's feasible in Wicket.