[ENH] Module product_pricelist_fixed:

    - Better view for pricelist items
    - Ensure values for base and base_ext (price type) are always in
      sync.

Based on launchpad, but including later improvement from Laetitia Gangloff

Launchpad:
https://code.launchpad.net/~therp-nl/openerp-product-attributes/7-0_fixed_price_extended/+merge/213378