Qt4: Hide buttons not matching typed command

There are way too many buttons on the extended command window.
Let the user type a letter or two, and hide the buttons that don't
match.

win/Qt4/qt4xcmd.h

@@ -21,6 +21,8 @@ public:
 
 private:
     QLabel *prompt;
+    QVector<QPushButton *> buttons;
+    void enableButtons();
 
 private slots:
     void cancel();