Fix handling of scroll bar clicks on Haiku

Fix handling of scroll bar clicks on Haiku

* src/haiku_support.cc (class EmacsView, BasicMouseDown)
(BasicMouseUp): Move MouseDown and MouseUp here.  New parameter
`scroll_bar'.
(MouseDown, MouseUp): Call basic variants.

(class EmacsScrollBar): New field `parent'.
(BScrollBar_make_for_view): Rename to
`be_create_scroll_bar_for_view'.
* src/haiku_support.h (struct haiku_button_event): New field
`scroll_bar'.
* src/haikuterm.c (haiku_scroll_bar_from_widget): Handle NULL
widget.
(haiku_scroll_bar_create): Update calls.
(haiku_mouse_position): Fix scroll bar part.
(haiku_read_socket): Handle button events on scroll bars as
scroll bar click events.