Approval multiple Workflow activities at once

I received a new required to allow users to accept/refuse multiple workflow activities by one clicking.

Modified 'TimeSheetApprovoal' user control to allow multiple selecting:

Now when I select 2 lines the button 'Actions' of workflow bar is hidden and multiple lines cannot be approved/rejected.

For one line the button is displayed:

Which ax classes  I have to modify to allow actions for multi lines select?

Where I can find some info about how Workflow works in Enterprise portal?