[REF] web, *: change dialog service api

* iap, web_tour

This commit changes the API of the dialog service.

Before, the API gave 2 functions: "open" and "close".
Now, there is only one function "add" which is the same as "open"
but returning a callback that does the close.

closes odoo-dev/odoo#940

Related: odoo-dev/enterprise#171
Signed-off-by: Géry Debongnie (ged) <ged@openerp.com>