Improve Xamarin implementation

[Zenrac]

Overview of the feature request

First Xamarin implementation (done in #489) lacks many important features.

Handle offline mode

• Generated App should be able to work even if the connection to DB fails.

Relation between entities

• Generated entity should be able to have relations with other entities.

Better iOS compatibility

• Improve UI for iOS

User management

• Allow to manage users on the application

Tests

• Unit tests
• Functional tests

Internationalization (i18n)

• Allow to use different language rather than only English.

Motivation for or Use Case

Xamarin is an interesting way to generate a mobile application with .NET. Its addition to JHipster is a very good thing but sadly it still need some features. Thankfully, those features can be easily added.

Related issues or PR

First Xamarin implementation done with #489

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