Lib edit tree: add read-only as a suffix

It's not ideal, because it will easily go off the right edge,
but the proper solution proably involves something involved
like little indicator icons in the tree or something.

This should be something of a clue at least some of the time.

eeschema/symbol_tree_synchronizing_adapter.cpp

@@ -270,6 +270,8 @@ void SYMBOL_TREE_SYNCHRONIZING_ADAPTER::GetValue( wxVariant& aVariant, wxDataVie
 
                 if( !m_libMgr->IsLibraryLoaded( node->m_Name ) )
                     aVariant = _( "(failed to load)" ) + wxS( " " ) + aVariant.GetString();
+                else if( m_libMgr->IsLibraryReadOnly( node->m_Name ) )
+                    aVariant = _( "(read-only)" ) + wxS( " " ) + aVariant.GetString();
             }
 
             const wxString& key = m_colIdxMap.at( aCol );