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.

src/gtk/checkbox.cpp

@@ -222,17 +222,17 @@ void wxCheckBox::SetLabel( const wxString& label )
     GTKSetLabelForLabel(GTK_LABEL(m_widgetLabel), label);
 }
 
-bool wxCheckBox::Enable( bool enable )
+void wxCheckBox::DoEnable(bool enable)
 {
-    if (!base_type::Enable(enable))
-        return false;
+    if ( !m_widgetLabel )
+        return;
+
+    base_type::DoEnable(enable);
 
     gtk_widget_set_sensitive( m_widgetLabel, enable );
 
     if (enable)
         GTKFixSensitivity();
-
-    return true;
 }
 
 void wxCheckBox::DoApplyWidgetStyle(GtkRcStyle *style)