Implement real menu help-echo text on Haiku

Implement real menu help-echo text on Haiku

* lisp/tooltip.el (tooltip-show-help): Remove Haiku-specific
conditional since that's now taken care of by C code.

* src/haiku_io.c (haiku_read_size):
(haiku_read_with_timeout):
(haiku_write_without_signal): Add parameter `popup_p'.  All
callers changed.
(port_popup_menu_to_emacs): New variable.

* src/haiku_support.cc (struct be_popup_menu_data): New
structure.
(be_popup_menu_thread_entry): New function.
(class EmacsMenuItem): New field `menu_ptr'.
(Highlight): Send help text to the popup port if this item
isn't for a menu bar.
(BMenu_add_item): Set menu_ptr appropriately.
(BMenu_run): Complete rewrite that allows to read help text from
the menu bar port.

* src/haiku_support.h (struct haiku_menu_bar_help_event): New
fields for popup menus.

* src/haikumenu.c (digest_menu_items): Only set help tooltip on
popup menus when system tooltips are enabled.
(haiku_menu_show_help):
(haiku_process_pending_signals_for_menu): New functions.
(haiku_menu_show): Pass new callbacks.