Ref T237, init caches from resource files (means the files delivered with swift)

* initFromLocalResourceFiles reads all local DB data
* made 2 functions virtual readFromJsonFiles, readFromJsonFilesInBackground

src/blackcore/db/icaodatareader.cpp

@@ -397,7 +397,7 @@ namespace BlackCore
 
         CStatusMessageList CIcaoDataReader::readFromJsonFiles(const QString &dir, CEntityFlags::Entity whatToRead)
         {
-            QDir directory(dir);
+            const QDir directory(dir);
             if (!directory.exists())
             {
                 return CStatusMessage(this).error("Missing directory '%1'") << dir;