The header glib_compat.h is now obsolete and can be removed.

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

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.

Obsoletes Tag::BeginAdd() and the complicated "bulk add" code.

Move to TagSettings.c and use C99 initializers.

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

Migrate the remaining callers to FatalError().

Replaces struct playlist_provider.

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

Reduce header dependencies.

Replaces GLib's GError.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Prepare for adding more code.

ReusableBuffer is more generic.