Reworked the way child properties can be added to a property that has not yet been...

Reworked the way child properties can be added to a property that has not yet been added to a grid or page; AddChild() deprecated, now use AddPrivateChild() instead. For public children, new member functions AppendChild() and InsertChild() should do the job.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59497 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775