Show header & footer only if requested (support for quiqqer/app)

The template should only show the header and footer if the corresponding attribute is set in the template.
This allows packages like quiqqer/app to hide the header on app requests.

Here is an example based the Business-Pro template: template-businesspro@e822b8d0

More information about the quiqqer/app support can be found here: app#33 (closed)