* doc/lispref/frames.texi (Raising and Lowering): Update
documentation to reflect that restacking frames is now
supported on Haiku.
* lisp/frame.el (haiku-frame-restack): New declaration.
(frame-restack): Use `haiku-frame-restack' on Haiku.
* src/haiku_support.cc (BWindow_send_behind):
* src/haiku_support.h (BWindow_send_behind):
* src/haikufns.c (Fhaiku_frame_restack): New functions.
(syms_of_haikufns): New subr `haiku-frame-restack'.
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