Media Browser add + button to add media to end of playlist

[rajlaud]

The request

The new media browser is a great way to load music. It would be nice to be able to add music to the end of a playlist too, rather than always replacing the currently playing music.

media_player.play_media takes an enqueue attribute that adds the music to the end of the playlist. How about a "+" button next to the play button that uses this feature to add media to the end of the playlist?

The alternatives

Not adding another button and continuing to use third-party software to load music. The downside of this feature request is potential visual clutter. But a small icon like a "+" would balance with the play icon and should be intuitive.

Additional information

Sonos and squeezebox support ATTR_ENQUEUE and are probably the main use case for this. (I'm testing code for browse media in squeezebox and will submit a PR soon).

Plex, roku, and kodi do not support ATTR_ENQUEUE and for video players it probably makes sense not to have it. I think there should be a SUPPORT_ENQUEUE flag added to the supported features bitmask so we know if to display the "+" or not.

If others think this makes sense I will start on a PR for the backend code.