Parser: Add guessSectionNameFromStrippedText() and refactor

Parser: Add guessSectionNameFromStrippedText() and refactor

Split up guessSectionNameFromWikiText() into pieces to reduce code
duplication, and provide guessSectionNameFromStrippedText() which
doesn't do link stripping.

Really these should be named guessSection*ANCHOR*From... because they
return an anchor (with encoding and a '#' prefix) instead of a section
name, but I didn't want to rename the existing one.

Also make normalizeSectionName static (it doesn't use $this) so that
guessSectionNameFromStrippedText() can be static as well.

Change-Id: I56b9dda805a51517549c5ed709f4bd747ca04577