feat: add icon selector demo.

demo/Ursa.Demo/TemplateSelectors/TimelineIconTemplateSelector.cs

@@ -0,0 +1,30 @@
+using Avalonia.Controls;
+using Avalonia.Controls.Shapes;
+using Avalonia.Controls.Templates;
+using Avalonia.Media;
+using Ursa.Controls;
+
+namespace Ursa.Demo.TemplateSelectors;
+
+public class TimelineIconTemplateSelector: ResourceDictionary, IDataTemplate
+{
+    
+    public Control? Build(object? param)
+    {
+        if (param is TimelineItemType t)
+        {
+            string s = t.ToString();
+            if (ContainsKey(s))
+            {
+                object? o = this[s];
+                if (o is Control c) return c;
+            }
+        }
+        return null;
+    }
+
+    public bool Match(object? data)
+    {
+        return data is TimelineItemType;
+    }
+}