Introduce Child-Notify

Introduce Child-Notify

Child-Notify replaces
  ChildClosed
  ChildRegistered
  ChildMoved
  ChildReplaced

as a pane that handles one typically needs to handle them all.
The difference notifications are reflected in ->num:

 1 : new child registered - ->focus is the child
-1 : existing child is being closed - ->focus is that child
-2 : existing child is being moved away - ->focus is tha child
 2 : existing child replaced - ->focus in the new child
     This follows a "-2" where the old was moved away.

Several handlers are extended to handle all possibilities.

Signed-off-by: NeilBrown <neil@brown.name>