Add project files.

UI/WPF/Markups/TriggerCollection.cs

@@ -0,0 +1,15 @@
+using System.Collections.ObjectModel;
+using TheXamlGuy.UI.WPF;
+
+namespace TheXamlGuy.UI.WPF;
+
+public class TriggerCollection : Collection<TriggerExtension>
+{
+    public void Add(object item)
+    {
+        if (item is TriggerExtension trigger)
+        {
+            base.Add(trigger);
+        }
+    }
+}