Tweak suppresion of word highlighting.

[de-vri-es]

This PR changes the way word highlighting is suppressed:

First, only a sequence of (left, right) will get word highlighting. A sequence of (left left right) or (left right right) will no longer be used for word highlighting.

As a consequence, the suppression of word highlighting based on the number of non-highlighted characters can be made more strict. Instead of suppressing it when highlighting is more than 1/3 of the line, it is now suppressed when highlighting is more than 2/3 of the line.

Open issues:

• The decision for highlight suppression takes into account unhighlighted whitespace too.
• It would be nice to try and turn sequences of left, left, left, right, right, right into left, right, left, right, left right if it results in small word diffs.