Cleanup KVO when view is deallocating

[krizpoon]

May cause crash when OAStackView was deallocated but still observing subview's hidden property.

[samsymons]

Is there anything I can do to help get this PR merged?

[nsomar]

@samsymons Hey, thanks for the interest.
Do you think you can get the travis build green again :) that would be super helpful!

[samsymons]

@oarrabi Absolutely, I'll take a look into that — thanks! 😄

[samsymons]

I have not yet had time to fix up the CI issues with this PR but hope to look into that this week; sorry about that!

I also found an edge case with this fix, where views added multiple times to the stack view will not get properly deallocated after this fix in dealloc:

[self removeObserverForViews:self.subviews];

In this case, I had a label being added to the stack view multiple times, but that line above will only remove the observer for subviews. I fixed this crash in our project by updating the fix to remove the observer for all arranged subviews, not just self.subviews.

It seems that addArrangedSubview: is assuming that it is always getting a new item, so you can have a situation where the same view is added multiple times. Does it makes sense to have addArrangedSubview: check whether the newly added subview already exists in the arranged subviews array before adding it?