Don't auto-lower when moving onto a menu bar on Haiku

Don't auto-lower when moving onto a menu bar on Haiku

* src/haiku_io.c (haiku_len): Handle `MENU_BAR_LEFT'.
* src/haiku_support.cc (EmacsMenuBar::MouseMoved): New function.
* src/haiku_support.h (enum haiku_event_type): New event
`MENU_BAR_LEFT'.
(struct haiku_menu_bar_left_event): New structure.
* src/haikuterm.c (haiku_read_socket): Don't auto-lower when the
mouse exits the frame view onto the menu bar and handle
MENU_BAR_LEFT events.