Layout manager refactoring

* ShoesLayout now gets a `Swt::App` right from the start, not magically a top_slot somewhere from which it then takes the dsl and the gui_app
* made what the layout does more understandable (hopefully) through some extracted methods
* removed the 100% duplicated `contents_alignment` method from layout. It was always called with the same arguments as the receiver, so I made it into an argument less instance method of slot :-)

Cheers,
Tobi