Re-add light entities and sync over MQTT

[koenhendriks]

The light entities are currently removed from registering. The lights should be added back and be able to sync with the actual state of the Button+ device over MQTT.

Single Light entity
There should only be a single light entity per button has the device does not support controller the front and back individually (see issue N. 64 @ https://button.plus/support/issues)

[roodbaard80]

It would be great if this can be added. I want to use this to show notifications and status of lights.

[koenhendriks]

There has been some documentation added on the button plus website about the LEDs:

https://button.plus/support/docs/96

[roodbaard80]

Ok, now the wait for firmware 1.12 starts :)