* Implementation of `state_template` for the Universal media_player
* add tracking to entities in state template
* use normal config_validation
* fix tests, use defaults in platform schema, remove extra keys
* and test the new option `state_template`
* lint fixes
* no need to check attributes against None
* use `async_added_to_hass` and call `async_track_state_change` from `hass.helpers`