Refactor diff code:

Refactor diff code:
 o Code clean-up and refactor.
 o Split the PhpWiki-independent part of the code into lib/difflib.php.
 o New CSS-formatted unified diff output replaces old table-heavy format.
 o Add character-level diffs.

Issues:
 o New CSS-formatted diffs are very ugly in NS4 (and probably other
   oldish browsers.)
 o Character-level diffs may be overkill.  Maybe back off to
   word-level diffs?

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@759 96ab9672-09ca-45d6-a79d-3d69d39ca109