Add `isLayoutKitView` for safe view removal

[inamiy]

It seems composite view's subviews e.g. UIButton's subviews (UIImageView and UIButtonLabel) gets removed and displays nothing when:

1.makeViews is called twice

2. ButtonLayout has viewReuseId (UIButton will be reused)

This is because purgeViews is removing views even though some of them are not managed by LayoutKit, so I added isLayoutKitView for safe view removal.

[codecov-io]

Current coverage is 92.75% (diff: 91.11%)

Merging #85 into master will increase coverage by 0.03%

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- Misses          217        219     +2   
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[nicksnyder]

Thanks for submitting, I should have a chance to look at this in a day or two.