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

Reduce the TagItem overhead.

Reduce heap allocator overhead.

For callers that already have a Tag instance.

Methods BeginAdd() and EndAdd() have been replaced by class
TagBuilder.

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

This is not a C++ class and must not be copied.

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.

Reduce header dependencies.