Move winId!=0 assertion from QWindow to QWidget

In "very basic platform plugins", the platform window ID can be null,
which conflicts with QWidget's special treatment of 0 as a parented widget.
Such is the case with EGLFS, which can return a null native window handle
for winId on some EGL implementations.

Move the assertion winId!=0 into the widget framework, as this assert is
not relevant for non-widget applications. A large proportion of basic
platform plugin users will see relief from this unnecessary assertion.

Change-Id: I25c9d96550cd747c77c9516d773e9cdebf7440ab
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>