Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

The old struct config_param remains only for top-level string options.

For type safety and code readability.

Reduce header dependencies.

Eliminate the temporary buffer, do I/O right into the WritableBuffer.

The "loop_count" configuration parameter allows the user to set how
many times a module with backward loops shall loop. "0" (the default)
means a module is not allowed to use backward loops at all. "-1"
enables inifinite looping.

Decouple some more from GLib.

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

Since Debian package 1:0.8.8.4-4, the pkg-config file does not contain
-I/usr/include/libmodplug anymore, and we need to add the
"libmodplug/" prefix to the #include line.

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