New variable haiku-pass-control-tab-to-system

New variable haiku-pass-control-tab-to-system

Allow passing C-TAB on to the Haiku operating system, fixing
window switching when an Emacs frame has focus (Bug#70138).
* src/haiku_support.cc (DispatchMessage): Conditionally pass
message to BWindow.
* src/haiku_support.h:
* src/haikufns.c (haiku_should_pass_control_tab_to_system): Add
variable haiku-pass-control-tab-to-system, and C function
haiku_should_pass_control_tab_to_system.

Co-authored-by: Po Lu <luangruo@yahoo.com>
Copyright-paperwork-exempt: yes