Indicate "keyboard focus" somehow in material browser and use it for delete

Material browser has now two separate selections, texture and material. There is no indication currently, which will be deleted if you press delete key (though it is always material currently). Delete should delete the item that was clicked last, and it should be highlighted differently from the other selected item. For example, the highlight rect line could be thicker.

This focus highlight should also only be active when keyboard focus in general is on the material browser view.