Animation of properties, circle menu extension fix

* Properties of objects can be animated with ease using the Animation
class. Submit a property to its constructor, add event listeners if
necessary, and call .start(). A key feature are constant durations,
independent from device performance.
* The new animation system also fixes a bug in the previous commit,
where the menu would only extend while the mouse was moving.
* Regarding the circle menu: it now has a closing animation as well.