Move code from the "mad" plugin.

Move code from the "mad" plugin.

Replaces GLib's g_str_has_prefix().

Replaces GLib's g_ascii_strcasecmp().

Decouple some more from GLib.

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

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

Replaces GLib's GError.

Avoid duplicating the tag.

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

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

These are confusing, and since MixRamp development has ceased, not
useful to anybody.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

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.