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 pull request #1012 from mirion/mirion_dev/flowed_layout

SC.FlowedLayout - when adding a child view, the layout is not calculated properly and the new child is positioned on top of others