This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Obsolete class CloseSongEnumerator, which was a kludge.

Type-safe wrapper for uri_get_suffix().

Convert filesystem charset to UTF-8 for playlist_list_open_uri().
This fixes one of many remaining charset bugs.

Replaces the method Close().

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Merge some duplicate code.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces struct playlist_provider.

Replaces GLib's GError.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.