Make menu bar key navigation work on Haiku

Make menu bar key navigation work on Haiku

* src/haiku_support.cc (menu_bar_active_p): New variable.
(DispatchMessage): Pass through key events if the menu bar is
active.
(MenusBeginning, MenusEnd): Set `menu_bar_active_p' according
to the state of the menu bar.
(BMenuBar_delete): Clear `menu_bar_active_p'.

* src/haikufns.c (haiku_free_frame_resources): Block input only
after checking that F is a window system frame.
* src/haikumenu.c (Fhaiku_menu_bar_open): Update doc string.