Cmd+M minimizes main window on Mac OS X (Closes #928216)

(cherry picked from commit dd7e515f9c)

src/mainwindow.h

@@ -110,6 +110,7 @@ protected slots:
   bool unlockUI();
   void notifyOfUpdate(QString);
   void showConnectionSettings();
+  void minimizeWindow();
   // Keyboard shortcuts
   void createKeyboardShortcuts();
   void displayTransferTab() const;