IsInitialized and IsActive are set too early

[mgnsm]

IsInitialized shouldn't be set to true before the OnInitializeAsync method has returned and the view model has actually been initialized.

Consider a view model that performs some asynchronous initialization:

public class SampleViewModel : Screen
{
    protected override async Task OnInitializeAsync(CancellationToken cancellationToken)
    {
        await Task.Delay(5000); //simulate doing something...
    }
}

And a view that displays a spinner or message to indicate that the view model is being initialized:

<TextBlock>
    <TextBlock.Style>
        <Style TargetType="TextBlock">
            <Setter Property="Text" Value="Ready!" />
            <Style.Triggers>
                <DataTrigger Binding="{Binding IsInitialized}" Value="False">
                    <Setter Property="Text" Value="Initializing..." />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

In version 4.0.52-alpha, the "Initializing..." message will never displayed because the IsInitialized property is set to true before the OnInitializeAsync method is awaited in the Screen class.

The same thing applies to IsActive and OnActivateAsync/OnDeactivateAsync.

These properties should be set once the tasks have completed. I'll submit another pull request.

[a44281071]

Shall we change it to Activating/Activated？Or before/after？
We can test this sample code SampleViewModel => SampleView : Window. Will the window immediately display?