* PanelEditor: fixes save/apply for undefined props in restoreModel

* Refactor: changes after PR comments

* Refactor: changes sourcePanel refresh strategy

* Added unit tests and minor refactoring of method, starting with cleanup, then setting properties from model

* Update public/app/features/dashboard/state/PanelModel.test.ts

Co-authored-by: Torkel Ödegaard <torkel@grafana.com>
Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>