Use `nullptr` instead of `0`.
GCC 4.8 compilation fixes.
Minor documentation fixes.

It's not needed to have empty `()` after class name in this type
of expressions:

    (new Object())
        ->setFoo(a)
        ->setBar(b);
Use `std::` prefix everywhere.
SceneGraph: this was fixed in 59a3a7029df0524d93fe5015113c668356da93ed.
SceneGraph: fully test also Object::setClean() for list of objects.

The test fails (it doesn't clean parent objects).
SceneGraph: fixed Object::setClean().

Now also all non-clean parent objects are cleaned.