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.

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

Finally allows loading playlist files on a storage plugin.  Commit
297e2747 attempted to implement this, but failed due to this bug.

Eliminates some overhead and some duplicate code, and fixes a serious
bug: the old code did not append the ".m3u" suffix, and thus the
"load" command was completely broken for stored playlists.  D'oh!

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

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Replaces struct playlist_provider.

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.

Memory leak fix.  The input_stream object passed to
playlist_list_open_stream_suffix() must be closed by the caller - this
however never happens in playlist_list_open_path(), because it does
not return it to the caller.