ValueTracker: turn value into a property, replace get_value / set_value

ValueTracker needs a refactor.
Quoting @leotrs :

there's many other things left to do. For example, get_value and set_value should all go away in favor of a property. And increment_value should be changed to also make use of that property, such that the code self.play(tracker.increment_value, 4.0) doesn't break.

AND THEN, there's the question of whether we even want self.play(tracker.increment_value, 4.0) to continue working at all. It's always been a mystery to me why sometimes you need ApplyMethod and not at other times.

In a nutshell :

• getting rid of get_value and set_value in favor of a property;
• Changing increment_value, maybe getting rid of it as we have now += operator for ValueTracker
• ?

Start by locating ValueTracker and all references to get_value, set_value, and increment_value. Read the surrounding animation usage, including self.play(tracker.increment_value, 4.0), and resolve whether that API should remain. Done means the property-based design and increment behavior are decided and all affected references are consistently updated.