Add button to expose -- Toggle Single

[afredsa]

Problem to solve

Mopidy (MPD) has the ability to play one song and stop. This does not seem to be exposed in IRIS. Not being able to see the state and/or change Toggle Single can cause the player behave in an unpredictable/unknown way until you use another front end that exposes the Single (ON/OFF) toggle to change the state.

Proposed solution

Add a toggle button next to the Random Button that toggles the single state.

Additional info

Here is the: Mopidy Set Single API

This the issue seen in Mopidy Discourse

This Feature may be related to #838