Improvements to Haiku selection handling

Improvements to Haiku selection handling

* lisp/term/haiku-win.el (haiku-selection-targets): Implement in Lisp.
* src/haiku_select.cc (be_get_message_type):
(be_set_message_type):
(be_get_message_message):
(be_add_message_message): New functions.

* src/haiku_support.cc (MessageReceived): Fix typo.
* src/haikuselect.c (haiku_selection_data_1)
(Fhaiku_selection_targets): Delete functions.
(haiku_message_to_lisp, lisp_to_type_code)
(haiku_lisp_to_message): Correctly decode and encode nested
messages, and fix encoding specially decoded types via numeric
names.  Also store and decode message types inside Lisp
messages.
(Fhaiku_drag_message): Update doc string.
(syms_of_haikuselect): Update subrs.
* src/haikuselect.h: Update prototypes.