Fixed inconsistent headerlayout

Headerlayout fixed with constant width and height of the logo which represents the title. Automatically scales with screen size. Code annotated with explanations. Also deleted setter method for appBarHeight which is never used, because there is no logic for it.

I think for now this is a good fix but the class app_settings.dart has a lot of dead code which will never be used because we limited the app to smartphone use without landscape mode. Maybe we should take a look at this later on.