Do not attempt to change colours of properties which are not present on the current page (in propgrid sample).

Test executed from 'Category Specific Colours' menu is designed to work only for specific category properties which are present only on 'Standard Items' page.
Display a message if current page is not this one and therefore test cannot be executed properly. Also make menu item non-checkable if it is accessed from improper page.

samples/propgrid/propgrid.h

@@ -195,6 +195,7 @@ public:
     void OnShowHeader( wxCommandEvent& event );
 #endif
     void OnDumpList( wxCommandEvent& event );
+    void OnCatColoursUpdateUI( wxUpdateUIEvent& event );
     void OnCatColours( wxCommandEvent& event );
     void OnSetColumns( wxCommandEvent& event );
     void OnMisc( wxCommandEvent& event );