Provide mute_toggle feature if device only supports mute & unmute

[zehnm]

I'm controlling my Denon network receiver with HA. Unfortunately it only provides mute and unmute commands, so toggling mute with the mute button on the remote is not possible.

Media-player entity tasks:

• verify if this old source comment in the integration is still true: // HASS media player doesn't support mute toggle!

It this is still true:

• handle mute_toggle in the integration, based on the current mute state
• provide mute toggle feature to the remote