so information about changed values is available in `afterSave`-event.

fixes #2892

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

- refactored elasticsearch AR to make type and index available in
  instantiate(), issue #1313

fixes #2281

Improved `QueryBuilder::buildLimit()` to support big numbers

fixes #1993

also filled framework CHANGELOG with relevant changes since alpha