Yet another global variable removed.

Move code from ReplayGainConfig.cxx.

Move code from replay_gain_get_real_mode().

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

Migrate the remaining callers to FatalError().

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

Fixes build problems with mingw32.

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

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.