osc: kitty notifications: cleanup and update to latest version of spec

* Don't store a list of unfinished notifications. Use a single one. If
  the notification ID of the 'current' notification doesn't match the
  previous, unfinished one, the 'current' notification replaces the
  previous one, instead of updating it.

* Update xstrjoin() to take an optional delimiter (for example ','),
  and use that when joining categories and 'alive IDs'.

* Rename ${action-arg} to ${action-argument}

* Update handling of the 'n' parameter (symbolic icon name); the spec
  allows it to be used multiple times, and the terminal is supposed to
  pick the first one it can resolve. Foot can't resolve icons at all,
  neither can 'notify-send' or 'fyi' (which is what foot typically
  executes to display a notification); it's the notification daemon that
  resolves icons.

  The spec _could_ be interpreted to mean the terminal should lookup
  .desktop files, and use the value of the 'Icon' key from the first
  matching .desktop files. But foot doesn't read .desktop files, and I
  don't intend to implement XDG directory scanning and parsing of
  .desktop files just to figure out which icon to use.

  Instead, use a simple heuristics; use the *shortest* symbolic
  names. The idea is pretty simple: plain icon names are typically
  shorter than .desktop file IDs.

terminal.h

@@ -799,9 +799,11 @@ struct terminal {
         void *cb_data;
     } shutdown;
 
-    /* Notifications that either haven't been sent yet, or have been
-       sent but not yet dismissed */
-    tll(struct notification) kitty_notifications;
+    /* State, to handle chunked notifications */
+    struct notification kitty_notification;
+
+    /* Currently active notifications, from foot's perspective (their
+       notification helper processes are still running) */
     tll(struct notification) active_notifications;
     struct notification_icon notification_icons[32];