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/

terminal.h

@@ -20,6 +20,7 @@
 #include "fdm.h"
 #include "key-binding.h"
 #include "macros.h"
+#include "notify.h"
 #include "reaper.h"
 #include "shm.h"
 #include "wayland.h"
@@ -798,6 +799,8 @@ struct terminal {
         void *cb_data;
     } shutdown;
 
+    tll(struct kitty_notification) kitty_notifications;
+
     char *foot_exe;
     char *cwd;