FIx the FlowedLayout. Replace the "childViews" observer by another one on "childViews.[]". This was necessary because when adding children to the FlowedLayout, the added children were not positioned correctly (the observer not being triggered).
Restore _scfl_childViewsDidChange for backward compatibility.
Remove the right childViews observer on destroyMixin
Merge branch 'mirion_dev/flowed_layout' of https://github.com/mirion/sproutcore into mirion_dev/select_view_exp