Add Engine.FocusGameViewport to implement game menu with camera focus gather

Source/Editor/Windows/GameWindow.cs

@@ -471,6 +471,23 @@ namespace FlaxEditor.Windows
             }
         }
 
+        /// <summary>
+        /// Focuses the game viewport. Shows the window if hidden or unselected.
+        /// </summary>
+        public void FocusGameViewport()
+        {
+            if (!IsDocked)
+            {
+                ShowFloating();
+            }
+            else if (!IsSelected)
+            {
+                SelectTab(false);
+                RootWindow?.Window?.Focus();
+            }
+            Focus();
+        }
+
         /// <summary>
         /// Takes the screenshot of the current viewport.
         /// </summary>