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.

The replay_gain_state struct holds the precalculated scale factor,
which is removed from struct replay_gain_info.

Don't log "ReplayGain is missing".

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The new command "replay_gain_mode" allows the user to switch the
replay gain mode on-the-fly.  No more mpd.conf editing.