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.

notify.c

@@ -19,7 +19,7 @@ notify_notify(const struct terminal *term, const char *title, const char *body)
 {
     LOG_DBG("notify: title=\"%s\", msg=\"%s\"", title, body);
 
-    if (term->kbd_focus) {
+    if (term->conf->notify_focus_inhibit && term->kbd_focus) {
         /* No notifications while we’re focused */
         return;
     }