add a tray icon stub to x11 platform.

src/Avalonia.X11/X11Platform.cs

@@ -103,7 +103,7 @@ namespace Avalonia.X11
 
         public ITrayIconImpl CreateTrayIcon ()
         {
-            throw new NotImplementedException();
+            return new X11TrayIconImpl();
         }
 
         public IWindowImpl CreateWindow()

src/Avalonia.X11/X11TrayIconImpl.cs

@@ -0,0 +1,33 @@
+using System;
+using Avalonia.Controls.Platform;
+using Avalonia.Platform;
+
+namespace Avalonia.X11
+{
+    class X11TrayIconImpl : ITrayIconImpl
+    {
+        public INativeMenuExporter MenuExporter => null;
+
+        public Action OnClicked { get; set; }
+
+        public void Dispose()
+        {
+            
+        }
+
+        public void SetIcon(IWindowIconImpl icon)
+        {
+            
+        }
+
+        public void SetIsVisible(bool visible)
+        {
+            
+        }
+
+        public void SetToolTipText(string text)
+        {
+            
+        }
+    }
+}