QPdfView requires a mandatory parent argument

Unlike all widgets, the parent argument of QPdfView is mandatory.

I don't know if this is a problem in C++, but at least in PyQt (and probably in PySide), when the widget is created in Designer, the `parent` keyworded argument is not accepted using the dynamic uic loader.

Even assuming that the problem only exists in PyQt, I still believe that the argument should not be mandatory, as it is clearly not required for the widget constructor: the fact that QPdfView can be created with a None (nullptr) parent, proves that it's unnecessary.

For the same reason, I think that the change should also be applied to QPdfDocument too.