Use content-core macro in templates

Use content-core instead of main macro for templates using the main_template's master macro. Among fitting better in the standard plone environment and prepares for displaying in a full view (some modifications needed).

Tests are failing, but I'm unsure if thats somehow related to my changes.

However, I pull-req this to see if there is some interest to merge it in. Tests fixes, if any, can come later on.