Rework registering / unregistering event handling

[AndrejMitrovic]

Currently we use operator overloads via:

label.onMouseEvent ~=
    (scope MouseEvent event)
    {
        if (event.action == MouseAction.press)
            button.toggle();
    };

This is cute, but it's not really obvious if we're appending to an array or a list or what is going on behind the scenes. From the user's POV, there's some questions:

• Can we unregister a handler?
• If yes, how? By providing the address of the delegate? Or just by having a unregisterHandler / clear function?
• Can you append multiple event handlers?
• If you can have multiple event handlers, can you delete each one?
• Should each event handler have a label ID? It might be more useful than using pointers (especially since we cannot serialize this form)

Some other issues:

• What about event handler persistence? There's event handlers defined in user-code, but what if we want to add new event handlers at runtime through user interaction? Maybe we should be able to persist them (make them serializable).