Added action listeners to all buttons

- added action listeners to all the item buttons - when clicked, a new
row is added to the Jtable and the total price indicator is updated
- added remove selected feature - when a row is selected, the remove
selected item button can be pressed to remove that item from the order
list. The total price indicator is updated after the item is removed
- discard order button removes all the items from the table and updates
the total price indicator