*: pos_discount,pos_loyalty,pos_restaurant,pos_self_order

In a previous commit (669b9fcc2fb82bb410e2b36e8bfa638ffd66cc2c) we added the
backend logic needed for creating combos in `pos`.

In this PR we add the combo functionality to the classic pos ui.

This means that products can now be grouped together and sold as a combo.

ex: Create a product called `Burger Menu` with type='combo'.This product will
have multiple combos associated with it, for ex:

- Drinks - will contain the list of drinks from which the customer can choose
- Main Course - will contain the list of main courses from which the customer
  can choose
- Dessert - will contain the list of desserts from which the customer can choose

In the event that one of the products inside one of the combos is to be more
expensive, this product will have a specific `combo_price` which will be added
to the main combo product's price.

We add the relational fields `combo_parent_id` and `combo_line_ids` in the
`pos.order.line` model. In the previous example, `combo_parent_id` will contain
the `id` of the `orderline` of the `Burger Menu` product.

In many cases `combo lines` have to be handled together. For example, deletion
of an orderline containing a product from a combo implies the deletion of all
other orderlines containing products from that combo. This means that multiple
methods from the codebase had to be changed for this reason. Places that needed
changes for this reason:

- `split_bill_screen.js`
- `addProductToCurrentOrder()`
- `remove_orderline()` and `add_orderline()` from the `Order` js model

In this PR we also:

- improve the demo data such that it contains a `pos combo`;
- refactor the `ProductItem` component such that it relies on a stateless
  component. This stateless component can now be used anywhere; ( it is now used
  in the popup which allows selecting the desired combo products )
- add a new method called `is_pos_groupable` on the `Orderline` js model. Before
  we relied on the product unit to see whether or not a product is groupable. We
  now needed a new mechanism, which could account for different reasons for
  which products are not groupable, such as belonging in a combo;

Note on price calculation:

- The idea is simple, imagine a rectangle divided into two parts. The whole
  rectangle is the total order, whereas the two partitions are the orderlines.
  - Now, imagine shrinking the rectangle by a certain factor. This shrinked
    rectangle represents an order containing a combo -- because normally, a
    combo results to smaller price. The two partitions are also shrinked by the
    same factor.
  - So the calculation will be:
    - Nominal total price is calculated. This is the sum of the orderlines'
      prices when they are added individually. This represents the whole
      rectangle.
    - Target price is derived from the combo product's price and selected
      components' combo prices. This represents the shrinked rectangle.
    - Reduction factor is computed as the ratio between the target price and the
      nominal price.
    - Each component's unit price is adjusted by multiplying the reduction
      factor.
    - When the unit prices of the components are computed, we now let the normal
      price computation take place.
- A spreadsheet is prepared to show a sample calculation:
  https://docs.google.com/spreadsheets/d/1iOdnaILnvKiaf2_z7jTijYiVfB_3mjKWjq6nOB70Un0/edit#gid=0

Task: 3430636

Co-authored-by: Joseph Caburnay <jcb@odoo.com>