Elm spa example header with shared button (on Click) attribute

[pdamoc]

@Nikolis_Galerakis

I was just avoiding to put to much on the Main because of fear it becomes to big and more complex than I can manage.

This is a good attitude. You usually have only one Main.elm and it pays to take extra care not to make it a magnet for things that would have ended up in other modules with more careful consideration.