Add an event so that users can detect when an Application icon is clicked

[danwalmsley]

On OSX when using ShutdownMode.OnExplicitShutdown, and the application has all its windows closed, the application icon remains in the Dock.

Most end-users expect that clicking the Dock icon will re-open the main window of the application. (See safari and other similar apps on MacOS).

However since there is no way for an Avalonia application to detect or handle this event, Avalonia applications can not implement this behaviour.

Applications can also not detect when the Dock icon is right clicked-> Hide or right clicked and Show... where the application is placed into a background state on macOS.

This PR adds an Application.Clicked event which only gets triggered on MacOS when the Dock icon (which represents the Application and not a Window) is clicked.

This PR adds an IActivatableLifetime that provides Activated and Deactivated events with event args providing the reason.

This api covers all bases for the specified scenarios on OSX and the api can be used as follows:

public override void OnFrameworkInitializationCompleted()
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
            {
                desktopLifetime.MainWindow = new MainWindow();
                desktopLifetime.ShutdownMode = ShutdownMode.OnExplicitShutdown;
            }

            if (ApplicationLifetime is IActivatableApplicationLifetime activatableApplicationLifetime)
            {
                activatableApplicationLifetime.Activated += ActivatableApplicationLifetimeOnActivated;
                
                activatableApplicationLifetime.Deactivated += ActivatableApplicationLifetimeOnDeactivated;
            }
        }

        private void ActivatableApplicationLifetimeOnActivated(object sender, ActivatedEventArgs e)
        {
            if (e.Kind == ActivationKind.Reopen)
            {
                if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
                {
                    if (desktopLifetime.Windows.Count == 0)
                    {
                        desktopLifetime.MainWindow = new MainWindow();
                        desktopLifetime.MainWindow.Show();
                    }
                }
            }
            else if (e.Kind == ActivationKind.OpenUri && e is ProtocolActivatedEventArgs pargs)
            {
                Console.WriteLine($"Open uri {pargs.Uri}");
            }
            else if (e.Kind == ActivationKind.Background)
            {
                Console.WriteLine("Unhidden");
            }
        }
        
        private void ActivatableApplicationLifetimeOnDeactivated(object sender, ActivatedEventArgs e)
        {
            if (e.Kind == ActivationKind.Background)
            {
                Console.WriteLine("Hidden");
            }
        }

This allows the developer to handle this in any way and implement the behaviour they wish for their application.

[thevortexcloud]

This PR adds an Application.Clicked

Should it be called something else so it's consistent with the rest of the Avalonia events? EG PointerPressed

[maxkatz6]

I think we are good with one of two possible APIs:

1. Something that follows UWP-like API:

public enum ActivationKind
{
    Reopen = 10, // Dock?
}
public class ActivatedEventArgs : EventArgs
{
    public ActivationKind Kind {get;}
}

public class Application
{
    public event EventHandler<ActivatedEventArgs> Activated;
}

2. Or MacOS.HandleReopenEvent attached event, specific to MacOS. And following Platform-specific APIs. #11384. This attached event also wouldn't be a big API blocker later, if we decide to use first approach instead at some point.

[workgroupengineering]

I think we are good with one of two possible APIs:

1. Something that follows UWP-like API:

public enum ActivationKind
{
    Reopen = 10, // Dock?
}
public class ActivatedEventArgs : EventArgs
{
    public ActivationKind Kind {get;}
}

public class Application
{
    public event EventHandler<ActivatedEventArgs> Activated;
}

I think it's the best choice, I think we should also migrate IApplicationPlatformEvents.

[maxkatz6]

@workgroupengineering yes, it would make sense there too

[danwalmsley]

@maxkatz6 @workgroupengineering, had an offline discussion with @maxkatz6,

will implement the above suggestion, but making it part of the application lifetime interface.

[danwalmsley]

Refactored using lifetimes and updated the description to show api usage.

[maxkatz6]

I feel like we should split these args, and add DeactivatedEventArgs instead of reusing. Maybe even with DeactivationKind.

[workgroupengineering]

If I interpreted correctly what @danwalmsley meant if ActivationKind.Background occurs in the Activated event, it indicates that the app has been reactivated from suspension, otherwise in Deactivated it indicates that the app has been suspended. So for each of the elements of the enum.

If so I would suggest renaming:
ActivatedEventArgs in ApplicationStateEventArgs
ActivationKind in ApplicationStateKind

and create

ApplicationStateActivatedEventArgs: ApplicationStateEventArgs
ApplicationStateDeactivatedEventArgs: ApplicationStateEventArgs

cc @robloo

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[avaloniaui-bot]

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