add missing signal declaration in Exposing Attributes to QML example

Task-number: QTBUG-33985
Change-Id: Id6a2a51467ee5de44bf51fb48e3f9346a832f8f2
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Stop polish-and-sync timer when the window is destroyed

The threaded render loop continues to run the polish-and-sync timer for
ever in case the QQuickWindow is destroyed while the timer is active.
Instead of pumping the timer events on the render thread for ever, the
timer has to be killed somewhere.

Change-Id: I8e458624e26ebfacb6cc69a4f2fbb3da57ae5e0e
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>