Add a convenience fonction to reverse the direction of the animation

It would be useful to have a way to easily change the direction of an animation, and still ensure that the direction of the animation is "Forward" when the animation is finished.

For a practical use case, have a look at the anomaly browser demo, file BrowserWindow.cpp. The functions "void BrowserWindow::keyReleaseEvent(QKeyEvent *event)" and "void BrowserWindow::animationFinished()" could be simpler with such a function.

Suggestions for the name:
::reverse()