Add support for OSC 52

Submitted by Raul

OSC 52 control code can be used to copy text into the clipboard.

tmux has native support for the OSC 52 sequence. See man tmux https://linux.die.net/man/1/tmux under the set-clipboard setting.

If gnome terminal could support OSC 52 it would remove the reliance on xclip or xsel and copying in tmux would work regardless of ssh tunnels.

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