Allow database plugins to announce that they have been modified.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Wrapper for pcm_resample_global_init(), just in case other PCM
libraries need initialization, too.

Otherwise sockets can't be created during IO thread initialization.

Reduce GLib usage.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

2 MB was too small for cross-fading a 24 bit file.  Increasing to 4 MB
is still not too large for weak machines, but is enough for
cross-fading.

Fixes crash.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

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

Add nullptr check, and return early from glue_db_init_and_load().

Move to TagSettings.c and use C99 initializers.

Allow using the Tag.cxx library without the Config library.

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

Replaces GLib's GError.

New GLib versions don't fail.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.