Add TMUX(1) support for `Window::copy`.
Add TMUX(1) support for `Window::copy`.

Refering to
http://permalink.gmane.org/gmane.comp.terminal-emulators.tmux.user/1324,
we learn there is a special form to by-pass TMUX(1) and send control
sequences to the upper terminal. We have to use the:

    \033Ptmux;…\033\\

control sequence where `…` is the original control sequence where `\033`
are doubled.

vcgato29/Central,vcgato29/Central