MODIFIED: Replace "faces styles" with a theme emulating PceEmacs

MODIFIED: Replace "faces styles" with a theme emulating PceEmacs

This adds a custom theme called 'sweeprolog-pce' that mimics the
highlighting of SWI-Prolog's built-in editor, PceEmacs.  It obsoletes
the "faces styles" feature that Sweep provided thus far.  For backward
compatibility, The user option 'sweeprolog-faces-style' is retained
and mostly still respected.

Thanks to Stefan Monnier for suggesting this approach for simplifying
Sweep's code and making it more conformant with Emacs conventions.

* sweeprolog-pce-theme.el: New file.

* sweeprolog.el: Remove 'sweeprolog-defface' macro its
'sweeprolog-*-face' functions, use plain face definitions and refer to
them directly by their names instead.  Also extend
'custom-theme-load-path' to include the new sweeprolog-pce-theme.el.

* README.org (Semantic Highlighting): Replace "Available Styles"
subsection with new subsection "PceEmacs Theme".