Support explicit translation ID and dotted namespaces in translation extraction (#9192)

Some translations, especially single words or other short
labels for buttons and the like, may not be transferable between
contexts even if they happen to be equal in English. In these cases,
setting an explicit translation ID is important for context separation.
Angular Translate also supports nested JSON in translation tables,
addressed using `.` as namespace separator; this enhancement makes use
of this when extracting translation with an explicit translation ID.

gui/default/syncthing/app.js

@@ -26,6 +26,7 @@ syncthing.config(function ($httpProvider, $translateProvider, LocaleServiceProvi
         prefix: 'assets/lang/lang-',
         suffix: '.json'
     });
+    $translateProvider.fallbackLanguage('en');
 
     LocaleServiceProvider.setAvailableLocales(validLangs);
     LocaleServiceProvider.setDefaultLocale('en');