Add player state and refactor inter task communication

[luar123]

This PR adds player state + callbacks and refactors the inter task communication.

• player state: IDLE, PLAYING or PAUSED
• callbacks can be registered and fire on state change
• player_pause() allows to pause the player from main app or any other task. Can be used to free i2s and play from another source
• player task is not aware of mute/volume state anymore. This simplifies player_send_snapcast_setting
• handle dac sleepmode from main task. Use state callback to decide wake up/ shutdown.
• dac is only unmuted if snapcast mute state and player mute state is false, avoiding unnecessary mute/unmute.
• discard chunks before decoding if player is paused
• use mutex instead of player_shutdown_in_progress from feat: Unified Ethernet interface with settings manager and web UI #201

I am unsure about the task notifications, had a race conditions when using them in the callbacks (that's why the main task callback uses a mutex). So maybe it would be better to use mutex or queues instead.

Todo: Either change notifications or set configTASK_NOTIFICATION_ARRAY_ENTRIES=2 in sdkconfigs.

@craigmillard86 Using this PR network_playback_stopped/network_playback_started from #201 could be replaced by a state callback.