Implement monitor change functions on Haiku

Implement monitor change functions on Haiku

* src/haiku_io.c (haiku_len): Handle new event type.
* src/haiku_support.cc (class EmacsScreenChangeMonitor): New
class.
(class Emacs, Emacs): Create new screen change monitor.
(DispatchMessage): Update fullscreen state if the screen
changed.
(SetFullscreen): Don't punt if fullscreen mode is identical.

* src/haiku_support.h (enum haiku_event_type): New event
`SCREEN_CHANGE_EVENT'.
(struct haiku_screen_changed_event): New struct.

* src/haikuterm.c (haiku_read_socket): Handle new event.