GTK has no magic facility for knowing that emacsclient
ran. Regardless, a terminal hook is not expensive, and I don't want to
add yet more window system typecases to the code. Terminal access
should be polymorphic. It's through terminal hooks that we make them
polymorphic. I'm not removing the terminal hook.
After thinking a bit, I figure that a better way to solve the problem
would be to document that window managers don't always respect
x-focus-frame, and to add a force parameter which makes it query for the
current server time and set it as the user time, thus making focus
setting more reliable.