tmux: add 'focusEvents'

With `tmux-sensible` being disabled by default, add an easy toggle to
enable this functionality.

Disabled by default, as in upstream `tmux`.

tests/modules/programs/tmux/default-shell.conf

@@ -19,6 +19,7 @@ set -g mode-keys   emacs
 
 
 set  -g mouse             off
+set  -g focus-events      off
 setw -g aggressive-resize off
 setw -g clock-mode-style  12
 set  -s escape-time       500