feat: wow, first prototype of navigation menu.

src/Ursa/Controls/Navigation/NavigationMenu.cs

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+using System.Collections;
+using System.Collections.Specialized;
+using Avalonia;
+using Avalonia.Collections;
+using Avalonia.Controls;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.Input;
+using Avalonia.Markup.Xaml.Templates;
+using Avalonia.Metadata;
+
+namespace Ursa.Controls;
+
+public class NavigationMenu: HeaderedSelectingItemsControl
+{
+    public static readonly StyledProperty<object?> FooterProperty = AvaloniaProperty.Register<NavigationMenu, object?>(
+        nameof(Footer));
+
+    public object? Footer
+    {
+        get => GetValue(FooterProperty);
+        set => SetValue(FooterProperty, value);
+    }
+
+    public static readonly StyledProperty<IDataTemplate> FooterTemplateProperty = AvaloniaProperty.Register<NavigationMenu, IDataTemplate>(
+        nameof(FooterTemplate));
+
+    public IDataTemplate FooterTemplate
+    {
+        get => GetValue(FooterTemplateProperty);
+        set => SetValue(FooterTemplateProperty, value);
+    }
+
+    public static readonly StyledProperty<object?> SelectedMenuItemProperty = AvaloniaProperty.Register<NavigationMenu, object?>(
+        nameof(SelectedMenuItem));
+
+    public object? SelectedMenuItem
+    {
+        get => GetValue(SelectedMenuItemProperty);
+        set => SetValue(SelectedMenuItemProperty, value);
+    }
+    
+    internal void UpdateSelection(NavigationMenuItem source)
+    {
+        var children = this.ItemsPanelRoot?.Children;
+        if (children is not null)
+        {
+            foreach (var child in children)
+            {
+                NavigationMenuItem? item = null;
+                if (child is NavigationMenuItem i)
+                {
+                    item = i;
+                }
+                else if (child is ContentPresenter { Child: NavigationMenuItem i2 })
+                {
+                    item = i2;
+                }
+                if (item != null)
+                {
+                    if(Equals(item, source)) continue;
+                    item.SetSelection(null, false, false);
+                }
+            }
+        }
+    }
+}

src/Ursa/Controls/Navigation/NavigationMenuItem.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Primitives;
+using Avalonia.Data;
+using Avalonia.Input;
+using Avalonia.LogicalTree;
+using Avalonia.Markup.Xaml.Templates;
+using Avalonia.Reactive;
+using Avalonia.VisualTree;
+
+namespace Ursa.Controls;
+
+[PseudoClasses(PC_Closed, PC_Selected, PC_Empty)]
+public class NavigationMenuItem: HeaderedSelectingItemsControl
+{
+    public const string PC_Closed = ":closed";
+    public const string PC_Selected = ":selected";
+    public const string PC_Empty = ":empty";
+
+    private NavigationMenu? _rootMenu;
+    private IDisposable? _ownerSubscription;
+    private IDisposable? _itemsBinding;
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        _rootMenu = this.FindAncestorOfType<NavigationMenu>();
+        if (ItemTemplate == null && _rootMenu?.ItemTemplate != null)
+        {
+            SetCurrentValue(ItemTemplateProperty, _rootMenu.ItemTemplate);
+        }
+        if (ItemContainerTheme == null && _rootMenu?.ItemContainerTheme != null)
+        {
+            SetCurrentValue(ItemContainerThemeProperty, _rootMenu.ItemContainerTheme);
+        }
+    }
+
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        base.OnPointerPressed(e);
+        // Leaf menu node, can be selected. 
+        if (this.ItemCount == 0)
+        {
+            var parents = this.GetSelfAndLogicalAncestors();
+            if (_rootMenu is not null && parents.Contains(_rootMenu))
+            {
+                object? o = this.DataContext ?? this;
+                _rootMenu.SelectedMenuItem = o;
+            }
+        }
+
+        e.Handled = true;
+        SetSelection(this, true, true);
+    }
+
+    internal void SetSelection(NavigationMenuItem? source, bool selected, bool propagateToParent = false)
+    {
+        this.PseudoClasses.Set(PC_Selected, selected);
+        var children = this.ItemsPanelRoot?.Children;
+        if (children is not null)
+        {
+            foreach (var child in children)
+            {
+                NavigationMenuItem? item = null;
+                if (child is NavigationMenuItem i)
+                {
+                    item = i;
+                }
+                else if (child is ContentPresenter { Child: NavigationMenuItem i2 })
+                {
+                    item = i2;
+                }
+                if (item != null)
+                {
+                    if(Equals(item, source)) continue;
+                    item.SetSelection(this, false, false);
+                }
+            }
+        }
+
+        if (propagateToParent)
+        {
+            var parent = this.FindAncestorOfType<NavigationMenuItem>();
+            if (parent != null)
+            {
+                parent.SetSelection(this, selected, true);
+            }
+            else
+            {
+                if (selected)
+                {
+                    _rootMenu?.UpdateSelection(this);
+                }
+            }
+        }
+    }
+}

src/Ursa/Controls/Navigation/NavigationMenuSeparator.cs

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+namespace Ursa.Controls;
+
+public class NavigationMenuSeparator: NavigationMenuItem
+{
+    
+}