Started implementing EventAggregator

src/Artemis.UI/Events/MainWindowFocusChangedEvent.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace Artemis.UI.Events
+{
+    public class MainWindowFocusChangedEvent : EventArgs
+    {
+        public MainWindowFocusChangedEvent(bool isFocused)
+        {
+            IsFocused = isFocused;
+        }
+
+        public bool IsFocused { get; }
+    }
+}