osc/notify: add support for OSC-99, kitty desktop notifications

This adds limited support for OSC-99, kitty desktop notifications[^1]. We
support everything defined by the "protocol", except:

* 'a': action to perform on notification activation. Since we don't
  trigger the notification ourselves (over D-Bus), we don't know a)
  which ID the notification got, or b) when it is clicked.
* ... and that's it. Everything else is supported

To be explicit, we *do* support:

* Chunked notifications (d=0|1), allowing the application to append
  data to a notification in chunks, before it's finally displayed.
* Plain UTF-8, or base64-encoded UTF-8 payload (e=0|1).
* Notification identifier (i=xyz).
* Payload type (p=title|body).
* When to honor the notification (o=always|unfocused|invisible), with
  the following quirks:
    - we don't know when the window is invisible, thus it's treated as
      'unfocused'.
    - the foot option 'notify-focus-inhibit' overrides 'always'
* Urgency (u=0|1|2)

[^1]: https://sw.kovidgoyal.net/kitty/desktop-notifications/

pgo/pgo.c

@@ -152,7 +152,9 @@ void ime_disable(struct seat *seat) {}
 void ime_reset_preedit(struct seat *seat) {}
 
 void
-notify_notify(const struct terminal *term, const char *title, const char *body)
+ notify_notify(
+     const struct terminal *term, const char *title, const char *body,
+     enum notify_when when, enum notify_urgency urgency)
 {
 }