Add option to control qBittorrent process memory priority

This is to avoid Windows swapping out other application data from
OS cache and put in torrent data which hinders other program
responsiveness.

The default value "Below normal" is choosen because casual users have
other higher priority apps (such as browser) running and they don't
expect OS to swap out its data. Dedicated seeders most probably will not
have other app running on their system and would expect qbt has
priority over other background services.

The option only has effect on Windows >= 8.

src/gui/advancedsettings.h

@@ -71,6 +71,8 @@ private:
     // OS dependent settings
 #if (defined(Q_OS_UNIX) && !defined(Q_OS_MACOS))
     QCheckBox m_checkBoxUseIconTheme;
+#elif defined(Q_OS_WIN)
+    QComboBox m_comboBoxOSMemoryPriority;
 #endif
 };