Make QSGNode and friends extensible via a d-pointer

Make QSGNode and friends extensible via a d-pointer

Convert the private void *m_reserved to a d_ptr so as to make QSGNode
inherited classes extensible without breaking binary compatibility.
QSGNode and subclasses do not create a private instance by default.

This is required by a follow-up commit where a new member needs to be
added to QSGSimpleTextureNode. Protected ctors have been added to the
superclasses of QSGSimpleTextureNode in the usual fashion.

Change-Id: I30c5f5d057654145d87f18c34c5d13a6ff5f7b11
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>