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_mac.mm

@@ -48,6 +48,10 @@ static const CGFloat kAtomWindowCornerRadius = 4.0;
   acceptsFirstMouse_ = accept;
 }
 
+- (void)windowDidBecomeMain:(NSNotification*)notification {
+  shell_->NotifyWindowFocus();
+}
+
 - (void)windowDidResignMain:(NSNotification*)notification {
   shell_->NotifyWindowBlur();
 }