Save log file in UTF-8 encoding

Otherwise it uses sytem defaults: usually UTF-8 on Linux and local 8-bit
encoding on Windows.

src/app/filelogger.cpp

@@ -115,6 +115,7 @@ void FileLogger::addLogMessage(const Log::Msg &msg)
     if (!m_logFile.isOpen()) return;
 
     QTextStream stream(&m_logFile);
+    stream.setCodec("UTF-8");
 
     switch (msg.type) {
     case Log::INFO: