Support animations which react to property changes while already running

Another suggestion for Qt 6: Qt Quick Animations don't react to property changes while they are running, making them a bit clumsy to use in declarative environment, where the binding evaluation order is often undefined or otherwise hard to control. In many cases you are forced to drop declarative bindings and just configure the animation imperatively within a function that ends with line that starts the animation (running = true).

There is probably some good reason why the animations work like this though, which I don't remember, but which should be checked e.g. from Michael Brasser or some other Qt Quick oldtimer to avoid throwing the baby out with the bath water.