[qtcontacts-sqlite] Improve DLG plugin interface. Contributes to JB#44742
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).