Implement `above' z-group on Haiku

Implement `above' z-group on Haiku

* src/haiku_support.cc (class EmacsWindow): New field `z_group'.
(RecomputeFeel): New function.
(ParentTo, BWindow_set_override_redirect): Use that instead
instead of manually juggling the window feel around.
(BWindow_set_z_group): New function.

* src/haiku_support.h (enum haiku_z_group): New enum.
* src/haikufns.c (haiku_set_parent_frame): Clean up coding
style.
(haiku_set_z_group): New function.
(haiku_create_frame): Always set z group after window creation,
like on X.
(haiku_frame_parm_handlers): Add `haiku_set_z_group'.