Fix -Woverloaded-virtual warnings about Window::Create()

Fix -Woverloaded-virtual warnings about Window::Create()

In gcc 13, -Wall turns on -Woverloaded-virtual

These virtual methods seem to be never actually used currently:

* PropPage-derived classes have a Create() which explicitly calls
PropPage::Create().

* PropPage::Create() and PropSheet::Create() do not call
Window::Create() (which is thus unused).

* We instantiate objects of Propage-derived and PropSheet types and
directly call their Create() method.

So simply fix the warning by removing the 'virtual' specifier.