Add project files.

UI/Avalonia/Markups/TriggerCollection.cs

@@ -0,0 +1,14 @@
+using System.Collections.ObjectModel;
+
+namespace TheXamlGuy.UI.Avalonia;
+
+public class TriggerCollection : Collection<Delegate>
+{
+    public void Add(object item)
+    {
+        if (item is Delegate trigger)
+        {
+            base.Add(trigger);
+        }
+    }
+}