QPointer : Add support for base class assignment.

QPointer currently does not support assignment of a subclass to a superclass, as in the following scenario :

struct Base : QObject {};
struct Derived : Base {};

QPointer<Derived> d = new Derived;
QPointer<Base> b = d; // error: conversion from 'QPointer<Derived>' to
non-scalar type 'QPointer<Base>' requested.

It would be a nice feature to have.