Spotify search and add to queue

db/index.php
* Add Spotify results to 'search' query output
* Make callback listeners for Spop add-to-queue commands

inc/player_lib.php
* Add Spotify search function which returns array of tracks matching query

js/volumio.playback.js
* Add callbacks for Spop add-to-queue commands

templates/indextpl.html
* Add context menu items for Spop add-to-queue