Simplify Haiku selection code

Simplify Haiku selection code

* src/haiku_select.cc (get_clipboard_object): New function.
(BClipboard_find_data, BClipboard_get_targets, BClipboard_set_data)
(BClipboard_find_system_data)
(BClipboard_find_primary_selection_data)
(BClipboard_find_secondary_selection_data)
(BClipboard_set_system_data, BClipboard_set_primary_selection_data)
(BClipboard_set_secondary_selection_data, BClipboard_free_data)
(BClipboard_system_targets, BClipboard_primary_targets)
(BClipboard_secondary_targets): Delete functions.
(be_find_clipboard_data_1, be_set_clipboard_data_1)
(be_get_clipboard_targets_1, be_find_clipboard_data)
(be_set_clipboard_data, be_get_clipboard_targets): New
functions.
(be_lock_clipboard_message, be_unlock_clipboard): Use
`get_clipboard_object' to get clipboard from ID.

* src/haikuselect.c (haiku_get_clipboard_name): New function.
(Fhaiku_selection_data, Fhaiku_selection_put)
(Fhaiku_selection_owner_p): Adjust to use new simplified
functions.

* src/haikuselect.h: Update prototypes.