Introduce support for Desktop Notifications on Haiku

Introduce support for Desktop Notifications on Haiku

* doc/lispref/os.texi (Desktop Notifications): Document Haiku
desktop notifications.

* etc/NEWS: Announce this change.

* lisp/org/org-clock.el (haiku-notifications-notify): New
declaration.
(org-show-notification): Employ that function.

* src/haiku_io.c (haiku_len) <NOTIFICATION_CLICK_EVENT>: Return
the length for this type of event.

* src/haiku_select.cc (my_team_id, be_display_notification): New
functions.

* src/haiku_support.cc (my_team_id, ArgvReceived): New
functions.

* src/haiku_support.h (enum haiku_event_type): New event type
NOTIFICATION_CLICK_EVENT.
(struct haiku_notification_click_event): New structure.

* src/haikuselect.c (haiku_notifications_notify_1)
(Fhaiku_notifications_notify): New functions.
(syms_of_haikuselect): Register new defsubr.

* src/haikuterm.c (haiku_read_socket):
* src/haikuselect.h:
* src/termhooks.h: Add new events for notification clicks on
Haiku.