Added a new Events object with :on() and :off() methods to start or
stop listening to a specific event respectively. This also allows to
add several callbacks for the same event.
Replaced the temporal Site.onChange & Sprite.onChange implementations.
Related to several issues (enable more possibilities for): #138, #1403, #1949, #2965, #2980
This should fix several crashes using objects in invalid ways where
the object doesn't exist anymore (e.g. an undo deleted referenced
objects in scripts).