elm_box/table: set the order directly when a container is registered

elm_box/table: set the order directly when a container is registered

The following situation:
 - A Box in a naviframe, with n children.
 - All added children register to the focus graph with the box as parent,
   order gets set correctly.
 - Naviframe hides this item, so box property tree unfocusable gets set
   to true, it gets unregistered from the focus graph, even every single
   child gets unregistered.
 - The item gets shown - every child and the table are getting
   registered again.
 - Order is not set again, since the box does not get changed
 - Order of the children is mixed up.

This should fix this case since the order is flushed every time the box
gets registered.