API calls to add/remove playlist entry

[jeroen]

There's been a few requests for API access to add/remove a playlist entry lately. One of them is outlines in attached forum thread.

The use cases are quite valid, and we should look into adding these options. In sum, people want to profit from the player's playlist features (repeating, prev/next buttons, playlist component) but not use the load() API call because it interrupts playback.

[jeroen]

As far as playlist functionality goes, we want to remain sparse. Much better to manage the playlist outside of the player then, since that provides more control.