Add new rules commerce_stock_decrease_by_line_item and commerce_stock_increase_by_line_item

Studying #1182384: Add to Cart button not disabled if more then one product per display I needed the ability to increase amounts as well as decrease.
Added new rules commerce_stock_decrease_by_line_item and commerce_stock_increase_by_line_item

The old commerce_stock_adjust_by_line_item has been retained as legacy (it's just a rehash of commerce_stock_decrease_by_line_item)