[REF] point_of_sale,*pos*: remove Backbone.js, single loading request

This commit accomplishes multiple objectives:

1. Loading of model data for the POS UI is now done in single
request (with some exemptions).
2. Backbone.js is removed and is replaced by reactivity.js
authored by the js framework team.
3. Better organization of assets in the `__manifest__.py`

Data Loading
------------

One request per model -- that's always been how the data is loaded
in POS UI. Now, we move the aggregation of data to the backend which
simplified the data loading. In the frontend, data loading is
initiated in `load_server_data` which makes the single rpc call
to the `load_pos_data` defined in `pos.session` model.

There remains some rpc calls during loading but the number is
greatly reduced.

This change result to faster loading of the POS UI. However,
there is no more progress bar during the loading.

Notable entry points:

- .js: `PosGlobalState.load_server_data`
- .py: `pos.session.load_pos_data`

It is possible to make customizations in the loading, but is done in
several steps:

1. Override `_pos_ui_models_to_load` to include the new model to load.
2. Define `_loader_params_<model_name>` to define the `search_params` and
optional `context`.
3. Define `_get_pos_ui_<model_name>` to return the data that will be
included in the full `loaded_data`. Perform the organization in this method
whenever necessary.
4. Override `_pos_data_process` for further post processing. The final
form of `loaded_data` in this method will be sent to the frontend.

Removal of Backbone.js
----------------------

POS is no longer dependent from `Backbone.js`, but it's replaced by a
new dependency -- `reactivity.js`. It's invented by the js framework
team which will be available in owl v2.

The consequences of this change are the following:

1. The old `PosModel` is renamed to `PosGlobalState`.
2. The name of the other models are kept (except Paymentline which
is renamed to Payment).
3. The name `PosModel` is now used as the base model of the data models.
This is symmetric to our use of `PosComponent`.
4. The instance of `PosGlobalState` is made reactive in `Chrome` (the
root component). As a result, whatever mutation made in that instance,
`Chrome` will rerender. But rendering is batched so multiple mutations
in a single sychronous function call will only result to a single
render call.
5. `PosModel`s can be extended in the `Registries` the same way as we
extend the `PosComponent`s.
```js
// E.g.
Registries.Component.extend(Chrome, PosResChrome);
Registries.Model.extend(Order, PosResOrder);
```

Reorganization of assets declaration
------------------------------------

This is a simple change that replaced the explicit enumeration
of loaded .js and .xml files. Also, we now have regions in the
manifest separating:

1. Augmentations of dependencies
2. Assets for `point_of_sale.index`
3. Assets for the qunit test (soon will be improved)

Other notable changes
---------------------

- Automatic call to `Order.save_to_db` is batched.
- Instances of `devices.DeviceProxy` (`proxy`), `devices.JobQueue`
(`proxy_queue`) and `BarcodeReader` (`barcode_reader`) are moved
to `point_of_sale.env`. Together with `posbus` and `posMutex`
(previously `flush_mutex`).
- `ActivePrograms` is now passed a props.
- `cashier` is no longer saved to `localStorage`.
- `pos_cache` is overhauled.
- `cache` is taken out from `PosDB` to prevent infinite rendering
loop whenever `cache` is mutated. It's now called `CACHE` in
the `db.js` file.
- reactive version of `posmodel` is exposed globally when in debug mode.
This means that calling when calling a mutator, the UI will react.
- Added control buttons in the `ProductScreen` is sorted during `Chrome`
setup. As a result, we don't need to worry on the loading order of the
control buttons components.
- Model can be simply instantiated using `BaseModel.create(obj)`. This
takes into account the extensions.
- `pos_coupon`: rewards are now updated whenever set_order is called.
- Component unit tests were removed. It wasn't useful and it just blocks
the development. We'll be introducing better unit testing later.

closes odoo/odoo#82461

Related: odoo/enterprise#23344
Signed-off-by: Masereel Pierre <pim@odoo.com>
Co-authored-by: Jacky (trj) <trj@odoo.com>