Fix RearrangeableListContainer<> crashing on replacement operations

[smoogipoo]

When handling range replacements, it first does a remove, then a sort, then an add, then a sort. The sort is intended to operate on drawable objects, but these do not yet exist because they haven't been added yet. In other words, the sort in-between the add and remove is causing the crash.

I've gone with the likely least-regressing path of making the sortItems() handle this case. I'd considered moving the events to the callback and doing a sort of "transaction", but there's quite a bit involved due to the sometimes-asynchronous operation of this container.
Maybe things could be better if we removed the asynchronous Add...

[bdach]

Maybe things could be better if we removed the asynchronous Add...

With the kinds of usages we've got of this client-side, I don't think we can.

[smoogipoo]

Yeah I mean I'm not even going to explore that route right now, it's working fine as it is minus this bug... Mentioned it in passing.

[bdach]

Have pushed another test because the one you did only exercised a full replace, I wanted to see if a partial replace correctly handles item order (and it does, as expected).

Seems good to me...?