feat: add splash window.

demo/Sandbox/Views/MainSplashWindow.axaml

@@ -0,0 +1,12 @@
+<u:SplashWindow 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:u="https://irihi.tech/ursa"
+        mc:Ignorable="d" d:DesignWidth="800"
+        d:DesignHeight="450"
+        CountDown="0:0:2"
+        x:Class="Sandbox.Views.MainSplashWindow"
+        Title="MainSplashWindow">
+    Welcome to Avalonia Splash Window!
+</u:SplashWindow>

demo/Sandbox/Views/MainSplashWindow.axaml.cs

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+using System;
+using System.Threading.Tasks;
+using System.Timers;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Avalonia.Threading;
+using Sandbox.ViewModels;
+using Ursa.Controls;
+
+namespace Sandbox.Views;
+
+public partial class MainSplashWindow : SplashWindow
+{
+    public MainSplashWindow()
+    {
+        InitializeComponent();
+    }
+    
+    protected override async Task<Window> CreateNextWindow()
+    {
+        return new MainWindow()
+        {
+            DataContext = new MainWindowViewModel()
+        };
+    }
+}