Models integration bases #40

This pull request introduces some noticeable changes to the current application.

Overall architecture

This pull request changes the architecture of the current application, by splitting it in four modules :

The Models module

The ShopNCook.Models where moved in an external module named Models.
This module contains the models structures (in Models)

Endpoint

The Endpoint Module contains interfaces for the ShopNCook application to use regardless of the real endpoint.
This allows us to make mocks for tests and stub implementation (c.f LocalEndpoint) during the application developpment.
The IEndpoint interface is the core structure of the Endpoint module.
This interface contains all the provided services, such as the Account Manager (for now).

When the application is started, the implementation of the endpoint is chosed by the App class in App.xaml.cs.

The LocalEndpoint module

The LocalEndpoint is an implementation of the Models.API interfaces which contains very simple hardcoded data and logic in order to simulate a server endpoint, which will replace the current endpoint implementation in the future.

Other additions, features

Some other minor changes

User notifications

The application currently has no way to send notifications (meaning a popup message) to the user.
This pull request introduces a UserNotifier interface, with a ConsoleUserNotifier implementation that prints the app's notices in the console. It'll be needed for a future PR to implement a notifier to show popups directly in the application.

Perfect, approved!