Remove redundant button clicked handler in QQuickMessageBox

Change c286b4fccb2d83fcc01b21913b95c5e4f21f2982 added the
QQuickAbstractMessageDialog::click() handler, which made the
QQuickMessageDialog::clicked() handler redundant.
(cherry-picked from qtdeclarative/a5669568904b37b1d6b297e7190d18306e198604)

Change-Id: Ifd770a3c67b35fd9a4929fc55b9e5298c36fb960
Task-number: QTBUG-35933
Reviewed-by: Liang Qi <liang.qi@digia.com>
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>