Stabilize tests using utilities from QQuickViewTestUtil.

Change QQuickViewTestUtil::createView() to center the window on
the screen to avoid taskbar areas. Add routine to move the mouse
away and use that in tests using QQuickViewTestUtil::flick()
and other mouse interaction since the mouse cursor can interfere
with it. Affected tests:  qquickgridview, qquickpathview,
qquickflickable, qquicklistview, qquickrepeater and qquickpositioners.

Task-number: QTBUG-33017
Change-Id: I540c0efb54a231dcb44c8fd5ad9573a2d4d4b9df
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>