Destroy child views in SwipeAltView on destroy().
In addChild, make sure the child is listed in this.children if we decide not to actually add it.

This is more reliable than checking child.parent, as sometimes if a view is re-rendered and its children has been reset.
Cleanup .on listener binding a bit.
Be more dilligent destroying children in layout views.
Fix multiple tap gestures by clearing this.active on touch/mouse up.
ScrollView: destroy children and clean up gesture listeners on destroy.
StackView: .destroy children, remove old StackView.