Various improvements to playlist and tuple system:
* Convert enums to C++ style.
* Add aud::range to allow enums to be used in range-based for loops.
* Abbreviate home folder with "~" in displayed file paths.
* Add concept of fallbacks to Tuple, replacing aud_playlist_entry_describe().
* Get song length and formatted title directly from the tuple.
* Make sure playlist scanning has stopped before shutting down the scanner.
* Do not require playlist scanning to finish before quitting.