[ADD] [7.0] 'pos_restaurant' module

This module add extra-features for point of Sale module related to restaurant.
For the time being, it adds:
- pos_table model to select the table of the customers;
- a field in pos_order to select the number of covers;

That features are available in back-end AND front-end views.

Nota:
- depends on pos_second_header module, not merged yet;
- This module has demo data, you can test it easily with populated database;

Flake8: OK.