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

src/Ursa/Controls/ToolBar/ToolBarPanel.cs

@@ -0,0 +1,34 @@
+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);
+        
+    }
+}