fixed Tree UI control to no longer corrupt child cache on item moving operations

[derammo]

Update code for children_cache was corrupting the cache when tree items are moved while the cache has not been built. This would lead to an array index error on subsequent use of the cache (since it would only have one item in it.)

The DEV_ENABLED safety code was agreed with @KoBeWi to stay in the code until refactoring work in this area is completed. It detects the bug fixed here and similar cache corruption. There are no unit tests for this, so this safety code will be useful until the class is again stable.

[trollodel]

That code in the move_* methods is wrong. Thanks for fixing it.
You cited the scene tree dock in the title, in what case did you trigger a bug using it? I don't think the tree there uses any of the method that creates the cache, nor the move_* methods.

[derammo]

Comment updated. Sorry for snarky response [I blame not enough coffee.] :)

[KoBeWi]

Looks ok.

I think I originally misunderstood your question about the validation; that code isn't really necessary. But it's ok to keep it.

[akien-mga]

Thanks!