Rewrite wikitext slicing for section edits

This commit adresses two issues:
  * rawWikiSlices used to drop the first byte of the suffix (usually a newline,
    but custom section edits may not be bound to newlines)
  * con used to insert way too much newlines: the newline that got
    dropped by the bug in rawWikiSlices and one additional newline in prefix
    and text each if they are not terminated by a newline. Now con only inserts
    newlines if there are absolutely no newlines between prefix and text and
    text and suffix.