Reduce reliance on MANDATORY_FIELDS and their implied order.

eeschema/tools/backannotate.cpp

@@ -527,8 +527,8 @@ void BACK_ANNOTATE::applyChangelist()
 
                     if( !m_dryRun )
                     {
-                        SCH_FIELD newField( symbol->GetPosition(), symbol->GetFieldCount(), symbol,
-                                            fpFieldName );
+                        SCH_FIELD newField( symbol->GetPosition(), symbol->GetNextFieldId(),
+                                            symbol, fpFieldName );
                         newField.SetText( fpFieldValue );
                         newField.SetVisible( false ); // Don't clutter up the schematic
                         symbol->AddField( newField );