Add support for child documents to docview framework.

Child documents are virtual documents corresponding to parts of their parent
document which can't be saved nor loaded independently of their parent and are
closed when the parent is closed.

This finally makes some use of wxDocument::m_documentParent field which was
always present in the docview code but never used before.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68051 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775