notify: add the notify-focus-inhibit config option

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.

config.h

@@ -238,6 +238,7 @@ struct config {
     } selection_target;
 
     struct config_spawn_template notify;
+    bool notify_focus_inhibit;
 
     struct {
         enum fcft_scaling_filter fcft_filter;