/// <summary>
/// Initalize modal service.
/// </summary>
/// <param name="serviceContainer"></param>
void InitalizeModalService(IUnityContainer serviceContainer)
{
// Setup navigation service.
var modalService = new ModalService(serviceContainer);
// WelcomePage
modalService.Register(typeof(WelcomeViewModel), typeof(WelcomePage));
// AboutPage
modalService.Register(typeof(AboutViewModel), typeof(AboutPage));
serviceContainer.RegisterInstance <IModalService>(modalService);
}