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

Replaces GLib's GError.

New GLib versions don't fail.

Deprecated.

Look up top-level config options by enum (= integer), not by name
string.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

.. and destruct the output object properly in test/run_output.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

Declaring the global variable _CRT_fmode is a documented hack.  What
an ugly platform!

Apply the replay gain in the output thread.  This means a new setting
will be active instantly, without going through the whole music pipe.
And we might have different replay gain settings for each audio output
device.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.