feat: add overlay dialog sample.

demo/Ursa.Demo/Dialogs/DialogWithAction.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:local="clr-namespace:Ursa.Demo.Dialogs"
+             x:DataType="local:DialogWithActionViewModel"
+             x:CompileBindings="True"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             x:Class="Ursa.Demo.Dialogs.DialogWithAction">
+    <StackPanel Margin="0 16 0 0">
+        <TextBlock Text="{Binding Title}"></TextBlock>
+        <Calendar SelectedDate="{Binding Date}" ></Calendar>
+        <StackPanel HorizontalAlignment="Right" Orientation="Horizontal" Spacing="20">
+            <Button Content="OK" Command="{Binding OKCommand}"></Button>
+            <Button Content="Cancel" Command="{Binding CancelCommand}"></Button>
+        </StackPanel>
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Dialogs/DialogWithAction.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class DialogWithAction : UserControl
+{
+    public DialogWithAction()
+    {
+        InitializeComponent();
+    }
+}

demo/Ursa.Demo/Dialogs/DialogWithActionViewModel.cs

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+using System;
+using System.Windows.Input;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using Ursa.Controls;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class DialogWithActionViewModel: ObservableObject, IDialogContext
+{
+    [ObservableProperty] private string _title;
+    [ObservableProperty] private DateTime _date;
+    public object? DefaultCloseResult { get; set; } = true;
+    public event EventHandler<object>? Closed;
+    
+    public ICommand OKCommand { get; set; }
+    public ICommand CancelCommand { get; set; }
+    
+    public DialogWithActionViewModel()
+    {
+        OKCommand = new RelayCommand(OK);
+        CancelCommand = new RelayCommand(Cancel);
+        Title = "Please select a date";
+        Date = DateTime.Now;
+    }
+
+    private void OK()
+    {
+        Closed?.Invoke(this, true);
+    }
+
+    private void Cancel()
+    {
+        Closed?.Invoke(this, false);
+    }
+}

demo/Ursa.Demo/Dialogs/PlainDialog.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             x:Class="Ursa.Demo.Dialogs.PlainDialog">
+    
+</UserControl>

demo/Ursa.Demo/Dialogs/PlainDialog.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Dialogs;
+
+public partial class PlainDialog : UserControl
+{
+    public PlainDialog()
+    {
+        InitializeComponent();
+    }
+}

demo/Ursa.Demo/Dialogs/PlainDialogViewModel.cs

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+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Ursa.Demo.Dialogs;
+
+public class PlainDialogViewModel: ObservableObject
+{
+    
+}