Implement cross-program drag-and-drop on Haiku

Implement cross-program drag-and-drop on Haiku

* doc/lispref/frames.texi (Drag and Drop): Fix documentation of
`x-begin-drag' to match actual function arity.
* lisp/term/haiku-win.el (haiku-dnd-selection-value): New
variable.
(haiku-dnd-selection-converters): New variable.
(haiku-dnd-convert-string): New function.
(gui-backend-get-selection, gui-backend-set-selection): Handle
XdndSelection specially.
(x-begin-drag): New function.

* src/haiku_select.cc (be_create_simple_message)
(be_add_message_data): New functions.
* src/haiku_support.cc (WAIT_FOR_RELEASE): New message type.
(class EmacsView, MouseUp): If waiting for release, reply and
drop event.
(be_drag_message, be_drag_message_thread_entry): New functions.
* src/haiku_support.h: Update prototypes.

* src/haikuselect.c (lisp_to_type_code, haiku_lisp_to_message)
(Fhaiku_drag_message): New functions.
(syms_of_haikuselect): Define new subr.

* src/haikuselect.h: Update prototypes.