[IMP] point_of_sale: prevent multicompany configuration issues

If the pos.config is not properly configured for multicompany (e.g. using location belonging to another company), launching a session with this config may fail (access rights).
This prevents to configure an incorrect point of sale in the first place.
[FIX] point_of_sale: incorrect sequences for orders

A new ir.sequence is generated at pos.config creation for the orders. However it was not used as the type was not set. The fallback was done on the general sequence.
In addition to the sequences being shared, it was not possible to create a pos.order in multicompany (no sequence found for user company, name was null).
Same issue for the pos.order.line.
This patch generates correctly pos.order and pos.order.line sequences at pos.config creation.

Instead of using the pos.config sequence to generate session number (not what this field was intended to), use the existing sequence for pos.session.
Remove company_id value on default pos.session sequence to make sure it's shared between companies and correctly set the prefix.
[FIX] account: name of bank statement is not required

At the confirmation of a bank statement, the name may not be set (e.g. generated by point of sale). This field is not requred so make a fallack on the statement line (which is required).
[IMP] doc/howtos/backend: improve views, fix workflow