Improve setSelected performance when large concept lists are in use

[stefandesu]

Selecting a new concept gets slower the larger the concept lists are. It's pretty much unnoticeable with "normal"-sized lists, but as soon as you get to a few thousand concepts, there's a significant delay between clicking a concept and it being selected (testing with all of ~8000 Thema concepts in a single list, I'd say the delay is between .5 and 1 second).

[stefandesu]

One idea I had was this:

1. Perform ALL modifications on items via modifyItem (this should in theory already be the case).
2. Add a system where components can ask to be notified only when a certain concept changes (this could be used in other parts of Cocoda as well).
3. Notify ItemList only about changes for concepts that are currently in view, and only update that part of the list. Other parts are irrelevant in that moment.

Not sure if this could work, but it's worth a try.