Add support for playing remote URIs

Here’s a branch which adds support for Aurena to play back remote URIs, both from the playlist and via the custom_file code. It modifies the database format slightly (partially deprecating the ‘paths’ table) and also modifies the URI format for /control/next so that it can accept full URIs as IDs (as well as traditional integer IDs).

It adds a dependency on GIO, but that shouldn’t be a problem since Aurena depended on a recent-enough version of GLib anyway.

It also amends the README to mention that Aurena is not to be considered secure, because it allows arbitrary URIs on the server to be sent to the clients.