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Methods BeginAdd() and EndAdd() have been replaced by class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

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

Convert a string into a tag_type enum.

Like tag_merge(), but can deal with NULL parameters, and frees both
tag objects.

This function has always been broken, but fortunately nobody used it.