allow selection and playback of any item in the playback queue

[mikedevnull]

Provide the backend web API to select any item from the playback queue as the current item for playback.
This should work in stopped state as well as during playback (i.e. stop current song and play newly selected song, but do nothing if current item didn't actually change)

• add item index selection to queue
• add selection to rest api
• implement playback control logic on selection change, i.e. playback actually follows selection
• event changes to playback queue and/or current playback item
• add selection to frontend api
• integrate frontend selection api into ui controls