Don't inherit from Qt private types

Do the d_ptr magic ourselves to avoid having to include private
headers from qtcore, qtgui and qtdeclarative.

It is hackish to hide QObject's d_ptr member to have the macros
working in a public class, but if anything goes wrong we just
need to replace the private macro convenience while maintaining
the binary compatibility of the stored extra opaque pointer.

Change-Id: Idb92f4f902826bef9068a5c2ef6ea31fc3fa15b2
Reviewed-by: Andras Becsi <andras.becsi@digia.com>