Add a focus event to BrowserWindow

- Tested in OSX
- Untested in GTK, but I expect it should work
- Did not see any similar constructs for notifications in Windows

atom/browser/native_window.h

@@ -177,6 +177,7 @@ class NativeWindow : public brightray::DefaultWebContentsDelegate,
   // related notifications.
   void NotifyWindowClosed();
   void NotifyWindowBlur();
+  void NotifyWindowFocus();
 
   void AddObserver(NativeWindowObserver* obs) {
     observers_.AddObserver(obs);