Move code from the Vorbis and FLAC decoder plugins.

Don't duplicate the buffer just to null-terminate the string.
According to libFLAC API documentation, the string is already
null-terminated.

To replace g_strdup().

Replaces GLib's g_ascii_strncasecmp().

Reduce heap allocator overhead.

Reduce header dependencies.

Allow reusing Tag instances.

The "bool" type is built-in.

Merge common code.

Use the new FLACMetadataIterator C++ class, which is more powerful.

This has been deprecated by the "embcue" playlist plugin.

Merge duplicate data.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.