Implement tray notifications for MS-Windows

Implement tray notifications for MS-Windows

* src/w32fns.c (MY_NOTIFYICONDATAW): New typedef.
(NOTIFYICONDATAW_V1_SIZE, NOTIFYICONDATAW_V2_SIZE)
(NOTIFYICONDATAW_V3_SIZE, NIF_INFO, NIIF_NONE, NIIF_INFO)
(NIIF_WARNING, NIIF_ERROR, EMACS_TRAY_NOTIFICATION_ID)
(EMACS_NOTIFICATION_MSG): New macros.
(NI_Severity): New enumeration.
(get_dll_version, utf8_mbslen_lim, add_tray_notification)
(delete_tray_notification, Fw32_notification_notify)
(Fw32_notification_close): New functions.
(syms_of_w32fns): Defsubr functions exposed to Lisp.  DEFSYM
keywords used by w32-notification-notify.

* doc/lispref/os.texi (Desktop Notifications): Describe the native
w32 tray notifications.