[FIX] web: wait for deferreds when actualizing mode

In edit mode, in a *2many with many2many_tags, when adding a new tag when none was set, discarding the form let the new tags displayed while it shouldn't (only a display issue, the tag wasn't added in database)

Fixes #2926
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32