Bug 6740: can add items at ordering/receiving but not remove

Bug 6740: can add items at ordering/receiving but not remove

The problem was, that the script was looking for the first
and second <a> tag in the code. When using plugins in the framework
this can't work. The patch changes the script to select the correct
<a> tags by using a class.

Also changes + and - to 'Add' and 'Delete' to make the meaning clearer
and clicking on them a bit easier.

To test:
1) AcqCreateItem = order
- Create a basket
- Create an order line
- Create more than one item
- Delete items
- Check quantity is calculated correctly
- Check items are created correctly

2) AcqCreateItem = receive
- Create basket
- Create 2 order lines, order >1 items
- Do a partial item by removing items from the receive form
- Receive all missing items
- Receive more items than ordered

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>