[idea] return $this in void method to allow chained calls

[remicollet]

Perhaps stupid, just an idea, to be able to write

$bos = new Box(Box::Vertical)
    ->setPadded()
    ->append(new Combo()
        ->append("Item 1")
        ->append("Item 2")
        ->append("Item 3"))
    ->append(new Separator(Separator::Horizontal))
    ...

Thus, not having to give a name to each UI item.