[libcalamares] Refactor timezone loading

- load from a TextStream. This is prep-work for alternate TZ data
  sources.

src/libcalamares/locale/TimeZone.cpp

@@ -110,12 +110,8 @@ RegionData::tr() const
 }
 
 static void
-loadTZData( RegionVector& regions, ZoneVector& zones )
-{
-    QFile file( TZ_DATA_FILE );
-    if ( file.open( QIODevice::ReadOnly | QIODevice::Text ) )
+loadTZData( RegionVector& regions, ZoneVector& zones, QTextStream& in )
 {
-        QTextStream in( &file );
     while ( !in.atEnd() )
     {
         QString line = in.readLine().trimmed().split( '#', SplitKeepEmptyParts ).first().trimmed();
@@ -186,6 +182,16 @@ loadTZData( RegionVector& regions, ZoneVector& zones )
         zones.append( new TimeZoneData( region, zone, countryCode, latitude, longitude ) );
     }
 }
+
+static void
+loadTZData( RegionVector& regions, ZoneVector& zones )
+{
+    QFile file( TZ_DATA_FILE );
+    if ( file.open( QIODevice::ReadOnly | QIODevice::Text ) )
+    {
+        QTextStream in( &file );
+        loadTZData( regions, zones, in );
+    }
 }