Deconflict some LIB_TREE_ITEM APIs so they can remain non-const.

(The non-const is required by FOOTPRINT_INFO, which must load
footrpints to get some of the data.)

cvpcb/cvpcb_mainframe.cpp

@@ -842,7 +842,7 @@ void CVPCB_MAINFRAME::DisplayStatus()
     if( fp )    // can be NULL if no netlist loaded
     {
         msg = wxString::Format( _( "Description: %s;  Keywords: %s" ),
-                                fp->GetDescription(),
+                                fp->GetDesc(),
                                 fp->GetKeywords() );
     }