Add QQuickCanvas::update()

We would like to make a distinction between QQuickItem::update()
which triggers updatePaintNode() to synchronize the scene
graph and requesting the toplevel QQuickCanvas to be repainted.

If we have this distinction in place it is possible for the
scene graph to detect that the scene is unchanged and avoid
rendering unless the user has explicitely asked for the
canvas to be repainted, which is often the case when hooking
into QQuickCanvas::beforeRender().

Change-Id: Ibe77f58423593deb217ef9f8082fa12009f45daf
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>