foot doesn't show desktop notifications (via OSC777) if the current
terminal has keyboard focus.
This is probably a sane default, but there are use cases where showing
a notification regardless of the focus status may be desired. For
example, a completion notification of a long running task inside a
non-focused tmux window.
This PR adds the notify-focus-inhibit option which can be used to
disable inhibition of notifications when the window has focus.
The default value is `yes`, which retains the old behavior.
