This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Enables using range-based "for".

This saves another 3% memory.

Fixes broken CUE sheet song tags (regression by commmit 7e8d254b).

Eliminate duplicate code.

Use class TagBuilder instead.

Replaces GLib's g_ascii_strcasecmp().

Methods BeginAdd() and EndAdd() have been replaced by class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

Move to TagSettings.c and use C99 initializers.

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

Migrate the remaining callers to FatalError().

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

Allow reusing Tag instances.

Eliminates explicit global initialisation.