Dialogs: avoid setting invalid geometry

QRect::setX() and setY() can change the width and height, even to a
negative value, which will make the dialog's intended geometry invalid.
The intention was that QQuickAbstractDialog::setX() and setY() should
move the window, so it should be done with QRect::moveLeft and moveTop.

Task-number: QTBUG-38898
Change-Id: I59104395ed39b09f6b7a99eb177820edb5e46cf6
Reviewed-by: Liang Qi <liang.qi@digia.com>