Use ConcurrentSkipListSet instead of CopyOnWriteArrayList for holding state keys

[digitalheir]

I was digging through the code and I a portion that might benefit from more efficient traversal.

Currently, stateKeys uses CopyOnWriteArrayList, presumably in order to iterate the list at that point while also removing entries from the list. dispose performs a linear search when disposing a screen, filtering all those entries which match the prefix of the screen key.

I reckon:

1. The search could be speeded up by taking advantage of the orderedness of strings
2. I thought the list might be better represented as a Set. I've noticed with the master-detail example, clicking an item and going back, that the list was polluted with entries for Screen#0, which should be conflated (right?).

I haven't done a complete implementation for KMM, just want to sketch out the idea and ask if it looks reasonable to you. Perhaps I am overlooking something, like a race condition you're more aware of than I am.

Let me know what you think!