feat: initialize, setup demo. make sure MVVM works.

src/Ursa/Controls/ToolBar/OverflowMode.cs

@@ -0,0 +1,8 @@
+namespace Ursa.Controls;
+
+public enum OverflowMode
+{
+    AsNeeded,
+    Always,
+    Never
+}

src/Ursa/Controls/ToolBar/ToolBar.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.Layout;
+
+namespace Ursa.Controls;
+
+[TemplatePart(PART_OverflowPanel, typeof(Panel))]
+public class ToolBar: HeaderedItemsControl
+{
+    public const string PART_OverflowPanel = "PART_OverflowPanel";
+    
+    internal Panel? OverflowPanel { get; private set; }
+    
+    private static readonly ITemplate<Panel?> DefaultTemplate =
+        new FuncTemplate<Panel?>(() => new ToolBarPanel() { Orientation = Orientation.Horizontal });
+    
+    public static readonly StyledProperty<Orientation> OrientationProperty =
+        StackPanel.OrientationProperty.AddOwner<ToolBar>();
+
+    public Orientation Orientation
+    {
+        get => GetValue(OrientationProperty);
+        set => SetValue(OrientationProperty, value);
+    }
+    
+    public static readonly StyledProperty<int> BandProperty = AvaloniaProperty.Register<ToolBar, int>(
+        nameof(Band));
+    
+    public int Band
+    {
+        get => GetValue(BandProperty);
+        set => SetValue(BandProperty, value);
+    }
+
+    static ToolBar()
+    {
+        IsTabStopProperty.OverrideDefaultValue<ToolBar>(false);
+        ItemsPanelProperty.OverrideDefaultValue<ToolBar>(DefaultTemplate);
+        OrientationProperty.OverrideDefaultValue<ToolBar>(Orientation.Horizontal);
+    }
+    
+    protected override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
+    {
+        return NeedsContainer<Control>(item, out recycleKey);
+    }
+
+    protected override Control CreateContainerForItemOverride(object? item, int index, object? recycleKey)
+    {
+        return new ContentPresenter();
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        OverflowPanel = e.NameScope.Find<Panel>(PART_OverflowPanel);
+    }
+}

src/Ursa/Controls/ToolBar/ToolBarOverflowPanel.cs

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+using Avalonia.Controls;
+
+namespace Ursa.Controls;
+
+public class ToolBarOverflowPanel: StackPanel
+{
+    
+}

src/Ursa/Controls/ToolBar/ToolBarPanel.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.LogicalTree;
+
+namespace Ursa.Controls;
+
+public class ToolBarPanel: StackPanel
+{
+    private ToolBar? _parent;
+    private Panel? _overflowPanel;
+
+    internal Panel? OverflowPanel => _overflowPanel ??= _parent?.OverflowPanel;
+    internal ToolBar? ParentToolBar => _parent ??= this.TemplatedParent as ToolBar;
+
+    static ToolBarPanel()
+    {
+        OrientationProperty.OverrideDefaultValue<ToolBarPanel>(Orientation.Horizontal);
+    }
+
+    protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToLogicalTree(e);
+        _parent = this.TemplatedParent as ToolBar;
+        if (_parent is null) return;
+        this[!OrientationProperty] = _parent[!OrientationProperty];
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        return base.MeasureOverride(availableSize);
+        
+    }
+}