[FIX] openerp/osv/fields: disable prefetching when reading inverse of one2many fields

This fixes issue #2146.  The inverse of a one2many field can be an inherited
field (_inherits).  In that case, we cannot read its value with a simple
database query.  Instead, we let the related field read it, but for performance
considerations we disable the prefetching of other fields.
[FIX] website.editor: need to set the range on start because the browser can be lost the caret and can't insert the media in the page.
[FIX] website_event_sale: need to use new api to send the context. Add some step for test to check if the cart have the distinct products
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64