custom field for BaseModel::load as fallback

- allows looking up models with different fields
- defaults to slug (previous behavior)
custom properties for BaseModel::_group()

- create Configurations of `source.property` of type Neon
  and provide list of values
- works nicely with $status and $type and custom properties
fix WidgetHelper template rendering for not-found widgets
updated Pages routing

- simplified Page-Lookup in routes
- throws DispatchException if called wrong