Since previous commit 70b67d1 procurements order are
not needed anymore see previous commit for more explanations.

The purpose of this commit is to handle code execution only in server
action and delegate schedule management to ir_cron model.

ir.cron model now inherits from ir.actions.server. Fields model, function
and args are removed as well as the logic to handle them. There is no
more code manipulation and evaluation in ir_cron, only a call to the run
method of ir.actions.server.

Cron form view use server action form view as primary view. This way
automated actions use the same base form as server actions with
cron details added.

Thanks to @fpodoo for the original idea and preliminary work. Thanks to
@jpr-odoo for first developments. Thanks to @jem-odoo for reviewing.