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/radiobox.h

@@ -141,6 +141,8 @@ protected:
     virtual void DoApplyWidgetStyle(GtkRcStyle *style) wxOVERRIDE;
     virtual GdkWindow *GTKGetWindow(wxArrayGdkWindows& windows) const wxOVERRIDE;
 
+    virtual void DoEnable(bool enable) wxOVERRIDE;
+
     virtual bool GTKNeedsToFilterSameWindowFocus() const wxOVERRIDE { return true; }
 
     virtual bool GTKWidgetNeedsMnemonic() const wxOVERRIDE;