Windows: improve file system mounting for customization

- Check if config.txt exists on drive letter before proceeding to
  write changes. Wait up to 3 seconds if not.
- Force drive letter assignment for portable SSDs and other devices
  that pretend to be a fixed drive. (Windows only auto-assigns
  drive letters for removable media).

Ref #171

downloadthread.h

@@ -176,6 +176,7 @@ protected:
 
 #ifdef Q_OS_WIN
     WinFile _file, _volumeFile;
+    QByteArray _nr;
 #elif defined(Q_OS_DARWIN)
     MacFile _file;
 #else