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.