Handling invalid widget schemas

[elbakerino]

There should be better handling of invalid schemas inside of widgets.

• e.g. the SimpleList does only work for type array with scalar-values as items
  • idea: should trigger "invalid" with SystemError as user info
• e.g. when no matched widget is found, but the value is invalid, it won't trigger invalidations
  • idea: switching to virtual rendering in that moment
• a schema validation in place would be too much
• ready to use schemas for backend/developer validation/testing would be great, e.g. running data tests against all written schemas