Use unique IDs for queue items

[FruitieX]

There is a little unambiguity in addressing queue items right now. It either has to be done by queue index (but this could change between eg. API requests), or by songID & backendName combination (but what if there are duplicates of a song?)

Ideally, we should generate a random GUID for a song each time it is added to the queue, and use this GUID in API calls where we address a certain song (or even position?) in the queue.

[FruitieX]

Needed so that e.g. calling "skip to song at pos 5" twice (in case of network lag) doesn't skip 10 songs

[FruitieX]

REST/socketio plugins as well as clients still need to implement & make use of this, then it can be marked as done.

[FruitieX]

On a second thought we should probably allow both ways of addressing positions, for very simple clients such as the terminal client.