Combine the two editor types to pick taxonomy terms for better UX experience

During discussions when working on HIPPLUG-1495 we realized that one new editor to replace both yet existing ones will help the UX experience of users much better.

We now have a tree picker (slightly improved with HIPPLUG-1495) and a Palette picker. This could be a combination of both:

That will make it much more clear how to select and deselect terms, and also show the tree structure of the terms. The tree area can also be filled with a list of all the leaf terms, as is now given by the Palette picker.