Big major overhaul to use always the same way everywhere how to fetch main-schema, behavior-schemas and its markers.
This was all very scrambled:
Sometime never took into account stuff from behaviors, or only from FTI based behaviors and not from IBehaviorAssignable adapter.
Also the caching was cleaned up. The tests are now better readable (at least i hope so).
While doing so plone.api style was followed as good as possible.
this fix reverts an accidentially introduced misconception due to lack of intended usage of schema vs. marker on behaviors
merge