Override DoEnable() instead of Enable() in wxGTK controls

This is slightly simpler, as it doesn't require checking whether the
control state really changes or not (it always does if DoEnable() is
called) and allows disabling the controls before creating them, e.g.
code like

    wxButton* const b = new wxButton();
    b->Disable();
    b->Create(this, wxID_OK);

works as expected now instead of spewing GTK+ errors.

include/wx/gtk/textctrl.h

@@ -95,7 +95,6 @@ public:
 
     // Overridden wxWindow methods
     virtual void SetWindowStyleFlag( long style ) wxOVERRIDE;
-    virtual bool Enable( bool enable = true ) wxOVERRIDE;
 
     // Implementation from now on
     void OnDropFiles( wxDropFilesEvent &event );
@@ -178,6 +177,8 @@ protected:
 private:
     void Init();
 
+    virtual void DoEnable(bool enable) wxOVERRIDE;
+
     // overridden wxTextEntry virtual methods
     virtual GtkEditable *GetEditable() const wxOVERRIDE;
     virtual GtkEntry *GetEntry() const wxOVERRIDE;