ApplicationHeader: Make top toolbars height more consistent
Make a consistent default height for all ApplicationHeader instances, which makes it not necessary anymore to override it all over the place
So, the hardcoded header height can now be dropped from the qml kcm infrastructure,
PageRowGlobalToolBarStyle preferredHeight is nowhardcoded 40, which comes from breeze but there isn't a way for now to get it in a less magical way.
This is the firat part of a global header refactor and simplification