Highlight recited sentence

Added highlighting to the sentence that is currently being recited,
using span elements. The highlighting visuals can be changed in
modules/ext.wikispeech.css. All text nodes that were used to generate
the utterance is wrapped in spans, except for the first and
last. These only get the ranges used as part of the utterance wrapped.

A path attribute was introduced to make the connection from an
utterance to the text nodes that were used to generate it. In doing
this, it was discovered that the position attributes, that were
previously added to both utterances and tokens, aren't used in this
solution. They were therefore removed.

Tests were changed a bit to have less overlap in what's being tested
and to make them easier to work with.

Bug: T148622
Change-Id: I12c8873265b7236c0677b25ce089bbb887ffdd9c