This commit improves the display label group plugin interface by
allowing plugins to report whether the specified locale is a
preferred locale for them, rather than merely valid, as some
plugins may work in multiple locales but should be preferentially
used (e.g. to determine the default display label groups) for
a subset of those locales.

For example, a pinyin grouping plugin might be valid in all locales
but preferred in locales which use Simplified Han writing script.

The database now populates the "default" supported display label
groups from the highest priority preferred plugin rather than
the highest priority valid plugin (falling back to highest priority
valid plugin if no preferred plugins exist for the current locale).