QQuickWindow: fix content item size

Resize content item in QQuickWindow::resizeEvent() instead of using
signals and slots. The signal-slot connections were only established
when child items were added in QML. It should work in C++ too, since
QQuickWindow and QQuickItem are part of the public C++ API and using
them in C++ is a perfectly valid use case.

Resizing the content item in resizeEvent() is not only faster than
using signals and slots, but also in theory a bit more flexible as
one would be able to override the event handler and implement their
own layouting.

Task-number: QTBUG-36938
Change-Id: Id05d4cf6d547021803050633e6f0a3359129a9f3
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>