[FIX] web_editor: correct ViewEditor widget life cycle
[FIX] web_editor: correct ViewEditor widget life cycle

These two fixes are now important since the ViewEditor is now also
used in the backend.

* The start method must return a deferred.
* The destroy method must unbind the event on the document element.

Note: in master, the ViewEditor is being refactored.

dfang/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo