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/checkbox.h

@@ -39,7 +39,6 @@ public:
     bool GetValue() const wxOVERRIDE;
 
     virtual void SetLabel( const wxString& label ) wxOVERRIDE;
-    virtual bool Enable( bool enable = true ) wxOVERRIDE;
 
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
@@ -52,6 +51,8 @@ protected:
     virtual void DoApplyWidgetStyle(GtkRcStyle *style) wxOVERRIDE;
     virtual GdkWindow *GTKGetWindow(wxArrayGdkWindows& windows) const wxOVERRIDE;
 
+    virtual void DoEnable(bool enable) wxOVERRIDE;
+
     void DoSet3StateValue(wxCheckBoxState state) wxOVERRIDE;
     wxCheckBoxState DoGet3StateValue() const wxOVERRIDE;