[FIX] point_of_sale: ignore TOTAlY returns

Returns were partially ignored when typing keystrokes, thanks to a return; when event which equaled 13, but the default behaviour (press on the focused input/button) was not prevented. This is now the case thanks to preventDefault. For instance, just after a discount set, the focused input was pressed and the associated value was added when scanning a new product with the scanner. Therefore, if we entered a discount of 30%, scanning a new product added '0', the last pushed button, to the discount, and then added the product.
[MERGE] Forward-port of latest 7.0 bugfixes, up to rev. 9684 rev-id: dle@openerp.com-20131209145652-3g9rgnfz1w8k0whw