Allow slowing animations

The QML tools already have a public way of slowing animations: the slow-animations argument. This just calls

QUnifiedTimer::instance()->setSlowModeEnabled(true);

Having a way of setting this for any Qt Quick application would be really useful in debugging animations.

My suggestion would be to have an enviroment variable; e.g. QT_QUICK_SLOW_ANIMATIONS. Even having a debug or info logging category that slows animations when enabled would be enough (which we already do in other places, publicly: https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph.html#scene-graph-and-rendering).