Perform keyboard switching on main thread.

Nothing except https://developer.apple.com/library/mac/technotes/tn2125/_index.html specifies that HIToolbox can't be run on the main thread, but switching keyboards seems like something that should.

I meant to put this in with my other 'fixes' branch, but seems like I forgot.
From `git blame` I see that @tiennou changed the keyboard switching code to run on a background thread. Can I ask why?

I'm still getting crashes when activating QS, and I think it's because of this