No more GUI code

The focus of this release is to remove GUI related code. Load event handling has been revised for more consistency. The API of the app class has changed significantly, no more handlers but setters. I hope the new doc will ease the client update process... See milestone 0.27.0 for details.

⚠️ Careful when updating, the API of the app class has changed quite a bit! It provides less handlers and more setters, also see the doc for more.