The loading of the action model extensions have been split in this
commit:
- when calling load, a first promise is returned. It is meant to be
resolved as soon as all the query informations are available.
- a second promise returned by a new method "isReady" is resolved when
all of the extension data have been fetched.

This difference allow to not wait for server calls irrelevant for the
query. By default "isReady" will always wait for a full load call, but
this is meant to be overridden in case optimisations can be made.

Refactoring of the Model class of `model.js` to support multiple
extensions.

The purpose is to build a model having the appropriate modules for a
given situation. For instance, we don't need the control panel model if
we don't have a control panel to start with.

Some logic of the control panel model (which was until now the "main"
search model) has been given to another model extension named
"Search Model Extension" which will synchronize the informations of all
models providing search data (domain, context, etc.).

The model instantiated in the actions/views is now an instance of the
"Action Model" (which directly extends the Model) and is meant to
replace the view model in the long term. It might however need some
improvements to hold such a role (partial interface rendering, etc.).

Part of task 2228968

Co-authored-by: Julien Mougenot <jum@odoo.com>
Co-authored-by: Mathieu Duckerts-Antoine <dam@odoo.com>