Add javadoc for ComponentEvent.getSource

[javier-godoy]

Consider this case:

    HorizontalLayout layout = new HorizontalLayout(new Checkbox());
    layout.addAttachListener(ev -> System.out.println(ev.getSource()));
    layout.setSizeFull();
    add(layout);

ComponentEvent.getSource just inherits its documentation from java.util.EventObject, which is

 @return the object on which the Event initially occurred

Which may be read as if the source is expected to be CheckBox (the click event initially occurred in a checkbox) instead of HorizontalLayout (which is the component where the listener was attached).