QWidget::paintEngine() returns 0 on QWS

On QWS, the implementation of QWidget::paintEngine() prints a warning message and then returns 0. This causes problems for paintEvent() functions that first check the features supported by the paint engine before painting a certain feature. Since this feature is supported on other platforms, and QWidget does have a valid paint engine, it should return a valid pointer.

At the very least, the fact that this function should not be called on QWS should be documented.