[FIX] website: restore customize menu loading
[FIX] website: restore customize menu loading

When opening the customize menu as soon as the page became visible, an
RPC crashed and the toggable options did not appear anymore. This
happened:

1) DOM is ready.
2) The user click on the menu.
3) At the same time, the customize menu is initialized (`attachTo`),
   its event handlers are bound and the async `willStart` method is
   called.
4) The event handler for the user click is called... and crashes as
   requires a variable which would have been set in the `start` method.

The bug did not appear in previous versions as the handler was not using
the standard system and bound the event handler "by hand" in the `start`
method.

Maybe `attachTo` should not bind event handlers before the `willStart`
method is completed; this would be a subject for the master branch. This
commit solves the problem by setting the required variable in the
`willStart` method instead of the `start` method.

closes odoo/odoo#28578
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo