ADDED: sweep Top-level Menu for listing Prolog top-levels

ADDED: sweep Top-level Menu for listing Prolog top-levels

* sweep.pl
- sweep_top_level_threads/2: new predicate for querying
currently active Prolog top-level threads

* sweeprolog.el
- sweeprolog-top-level-menu--entries: new function, exposing
sweep_top_level_threads/2 to Elisp.
- sweeprolog-top-level-menu-mode: new major mode derived from
tabulated-list-mode
- sweeprolog-top-level-menu-go-to: new command, switches to the
top-level buffer of the Top-level Menu entry at point
- sweeprolog-top-level-menu-kill: new command, kills the top-level
buffer of the Top-level Menu entry at point
- sweeprolog-top-level-menu-new: new command, creates a new top-level
from the Top-level Menu
- sweeprolog-list-top-levels: new command, opens the Top-level Menu