[AUCTeX] Some font locking changes

[Ralf Angeli]

Currently I am working on font-latex to achieve stuff like in the
following screenshot:

In the screenshot you can see that unfinished macros and environments
are no longer bleeding all over the place but rather get an indicator
in warning face that something's wrong.  For LaTeX macros the
correctness of the order and type of arguments is checked as well.
You can also see that braces and brackets are no longer highlighted.

What you cannot see is that the changes might make multiline font
locking possible on XEmacs as well.  But this needs still some
additional work.

The new code is already usable (excluding XEmacs) but the changes are
quite extensive (the diff file is about 1200 lines at the moment).  In
order for it to get some testing I'd like to put it into a branch.  If
nobody objects I'll create the branch in due time.

-- 
Ralf