feat: add loading.

demo/Ursa.Demo/Pages/LoadingDemo.axaml

@@ -0,0 +1,25 @@
+<UserControl
+    x:Class="Ursa.Demo.Pages.LoadingDemo"
+    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"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mc:Ignorable="d">
+    <Grid RowDefinitions="Auto, Auto, *">
+        <ToggleSwitch Name="s" Content="Loading" />
+        <u:LoadingIcon Grid.Row="1" />
+        <Panel
+            Grid.Row="2"
+            HorizontalAlignment="Stretch"
+            VerticalAlignment="Stretch">
+            <u:Banner
+                HorizontalAlignment="Center"
+                VerticalAlignment="Center"
+                Content="Hello Ursa!" />
+            <u:Loading Content="Loading..." IsLoading="{Binding #s.IsChecked}" />
+        </Panel>
+    </Grid>
+</UserControl>

demo/Ursa.Demo/Pages/LoadingDemo.axaml.cs

@@ -0,0 +1,18 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class LoadingDemo : UserControl
+{
+    public LoadingDemo()
+    {
+        InitializeComponent();
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}