Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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().