New visible-bell for NextStep (OS X El Capitan compatible).
Instead of inverting a rectangle in the middle of the frame, use
the standard NextStep image "caution", represented using an
warning sign with an exclamation mark. (Bug#21662)
Implemented based on a suggestion drafted by Mustafa Kocaturk.
* src/nsterm.m (EmacsBell): New class for managing the caution
image. Support multiple active bells, the image is removed once
all bells have timed out.
(ns_timeout): Removed, no longer used.
(ns_ring_bell): Reimplemented to use EmacsBell.