Use the same "changed" GTK callback for wxComboBox and wxTextCtrl

And reuse EnableTextChangedEvents() between these classes as well.

No real changes so far, this is just a refactoring to centralize the
code in a single place before modifying it.

include/wx/gtk/combobox.h

@@ -151,7 +151,6 @@ private:
     // From wxTextEntry:
     virtual wxWindow *GetEditableWindow() wxOVERRIDE { return this; }
     virtual GtkEditable *GetEditable() const wxOVERRIDE;
-    virtual void EnableTextChangedEvents(bool enable) wxOVERRIDE;
 
     void Init();