Improve translation look-up logic. This properly handles cases where a translation is available for the language but not for the specific territory (ie, "es_AR" now falls back on "es" not the default "en_US").
- use wildcard for TRANSLATIONS in bitcoin-qt.pro to automatically build all translations present in src/qt/locale (thanks @tcatm)
- first load translations/<language>.qm, then translations/<language>_<TERRITORY>.qm, so that territory-specific translations take precedence, but the fallback is on the base language if no territory-specific translation exists