after-new-child in panel:dragable-mixin

This method is called after a new containee area is created with this area as its container. The new child is provided as an argument to the method.

• (send a-panel:dragable-mixin after-new-child child void
     child : (instance-of (implements area<%>))

  Updates the number of percentages to make sure that it matches the number of children and calls after-percentage-change.