Add uimage command to get cover image of a track or album

To get a cover image of a track the track, the album and the image must be loaded. libspotify might load these data asynchronous. This could lead some headache.

I've tried to get the async loading/polling/waiting as readable as possible with extracted methods `_uri_image_cb_*()`, introduced callback data struct `uri_image_cb_data`, and enum `uri_image_cb_result_type`. Further, I've tried to avoid `goto` statements. So the new code is similar to existing uri commands but different. Hope this is fine.

Please add comments if any changes should be done before merging.