Create client-side API for Wayland extensions

The new Qt Compositor API contains convenience classes for creating Wayland extensions. On the client side, the API is low-level, and a new platform plugin must be created for each combination of plugins.

Therefore, we should:

1. Create a high-level API for implementing extensions client-side
2. Have a generic plugin mechanism for extensions

We already have specific plugins for the wayland platform plugin: hardware integration, shell integration...

The generic mechanism should load plugins on demand: try to load an extension plugin when the compositor asks for an unknown extension.

The plugins should also be available through QWaylandNativeInterface, somehow.