Merge some duplicate code.

Replaces g_uri_parse_scheme().

Decouple some more from GLib.

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

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.

Migrate the remaining callers to FatalError().

Replaces struct playlist_provider.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Hide the definition from C code, to prepare the transition to C++.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

Requires YAJL to build, and this doesn't include the necessary
automake changes. Can be built using
./configure CFLAGS="-I/usr/include/yajl" LIBS="-lyajl" --enable-soundcloud

Add the following to your config:

playlist_plugin {
        name "soundcloud"
        enabled "true"
        apikey "c4c979fd6f241b5b30431d722af212e8"
}

Then you can stream from soundcloud using calls like:

mpc load soundcloud://track/<track-id>
mpc load soundcloud://playlist/<playlist-id>
mpc load soundcloud://url/http://soundcloud.com/some/track/or/playlist

For the last case, you can leave off the http:// or
http://soundcloud.com/ .

The FLAC playlist plugin has been superseded by the "embcue" playlist
plugin, which can read the embedded CUE sheets of all formats.

Parses CUE data from the "CUESHEET" tag.  Needs further integration in
the update thread.

To replace libcue, the unmaintained and crashy library.

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.

For spotify playlists or tracks. Uses a spt uri, so with mpc you can
add playlists with

  mpc load spt://spotify:user:simon.kagstrom:playlist:3SUwkOe5VbVHysZcidEZtH

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

If the MIME type of a stream contains a semicolon, remove it and the
text after it to probe for a playlist plugin.

Merged both loops into playlist_list_open_stream().  This is needed
because playlist_list_open_stream() needs to know the MIME type, which
is only known after the stream has become "ready".