Implement "Add Selected" button in the add-remove part function


The user experience for the Add / Remove function was improved by adding a button called Add Selected where the user can select multiple parts and click the button to build up the item.  Before, the interface was not intuitive because the user had to double-click the items to add parts and it was confusing.  Now, you can just multi-select parts and click the Add Selected button to bulk insert parts.