It's not a perfect solution but you can log into your Hangouts account on OS X's Messages app, so you can largely combine iMessage and Hangouts on your desktop in one app if you use both.

I'm often surprised how many people don't know that Messages semi-supports Hangouts (not implying that you don't)..

Obviously things like video chat don't work, so it's more like what Google Talk used to be, but it's better than keeping 2 apps open.