Arena-py

Changed
- Queue all incoming MQTT messages for processing and remove `network_loop_interval`.

Added
- Add `end_program_callback`, which is called whenever client disconnects.

Changed
- Remove `Color` in `Material` deprecation.
- Fix undeclared variable `password` bug.

Changed
- Async MQTT loop cancellation bug fixes.

Changed
- Event loop bug fixes.

Added
- `on_msg_callback` receives __all__ messages, including duplicates from other callbacks.
- Object children now have the class variable `object_type` which is the name of the object_type for the class.

Changed
- Library calls `get_persisted_objs` on connect to cache persisted objects.
- Object instances now all have `clickable` property which checks if the Object is clickable.
- Improved event loop using more advanced asyncio for MQTT client loop and message processing.

Added
- Use `scene.get_persisted_objs()` to get all persisted objects in a scene.

Changed
- Improved message parsing and Object creation whenever a message arrives.
- Callbacks and event handlers now take __three__ arguments (`scene`, `obj`/`evt`, `msg`). `scene` is a reference to the scene.
`obj`/`evt` is an `Object` or `Event` (depending on the callback/handler). `msg` is the raw JSON message if needed by user.