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

demo/Ursa.Demo/DataTemplates/MenuDataTemplateSelector.cs

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+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Converters;
+
+public class MenuDataTemplateSelector: IDataTemplate
+{
+    public IDataTemplate? MenuTemplate { get; set; }
+    public IDataTemplate? SeparatorTemplate { get; set; }
+    
+    public Control? Build(object? param)
+    {
+        if (param is MenuItemViewModel vm)
+        {
+            if (vm.IsSeparator) return SeparatorTemplate?.Build(vm);
+            else return MenuTemplate?.Build(vm);
+        }
+        return null;
+    }
+
+    public bool Match(object? data)
+    {
+        return true;
+    }
+}

demo/Ursa.Demo/DataTemplates/ToolBarItemTemplateSelector.cs

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+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.Data;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Converters;
+
+public class ToolBarItemTemplateSelector: IDataTemplate
+{
+    public static ToolBarItemTemplateSelector Instance { get; } = new();
+    public Control? Build(object? param)
+    {
+        if (param is null) return null;
+        if (param is ToolBarButtonItemViewModel vm)
+        {
+            return new Button()
+            {
+                [!ContentControl.ContentProperty] = new Binding() { Path = "Content" },
+                [!Button.CommandProperty] = new Binding() { Path = "Command" },
+            };
+        }
+        if (param is ToolBarCheckBoxItemViweModel cb)
+        {
+            return new CheckBox()
+            {
+                [!ContentControl.ContentProperty] = new Binding() { Path = "Content" },
+                [!ToggleButton.IsCheckedProperty] = new Binding() { Path = "IsChecked" },
+                [!Button.CommandProperty] = new Binding() { Path = "Command" },
+            };
+        }
+        if (param is ToolBarComboBoxItemViewModel combo)
+        {
+            return new ComboBox()
+            {
+                [!ContentControl.ContentProperty] = new Binding() { Path = "Content" },
+                [!SelectingItemsControl.SelectedItemProperty] = new Binding() { Path = "SelectedItem" },
+                [!ItemsControl.ItemsSourceProperty] = new Binding() { Path = "Items" },
+            };
+        }
+        return new Button() { Content = "Undefined Item" };
+    }
+
+    public bool Match(object? data)
+    {
+        return data is ToolBarItemViewModel;
+    }
+}

demo/Ursa.Demo/DataTemplates/ViewLocator.cs

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+using System;
+using Avalonia.Controls;
+using Avalonia.Controls.Templates;
+using Ursa.Demo.Pages;
+
+namespace Ursa.Demo.Converters;
+
+public class ViewLocator: IDataTemplate
+{
+    public Control? Build(object? param)
+    {
+        if (param is null) return null;
+        var name = param.GetType().Name!.Replace("ViewModel", "");
+        var type = Type.GetType("Ursa.Demo.Pages."+name);
+        if (type != null)
+        {
+            return (Control)Activator.CreateInstance(type)!;
+        }
+        else
+        {
+            return new TextBlock { Text = "Not Found: " + name };
+        }
+    }
+
+    public bool Match(object? data)
+    {
+        return true;
+    }
+}