DockableVDividedBox and DockableHDividedBox can be used just as
regular DividedBox in the MXML file.
But do not use them in actionscript file at run time, because they
will be created and removed automaticly
according to the layout of the DockablePanel instances.
public function DockableVDividedBox()
public override function removeChild(child:DisplayObject):DisplayObject
If there is only one child after the removing.
The DockableVDividedBox itself will be removed, and its child
will be add to its parent.
Parameters
Returns