Extract ODF field highlighting into standalone view

As per design discussed in issue #816. Extracting this into a separate view makes it optional for consumers to use (or potentially further customize).

Fixes #816.

For reviewers, the best way to review this is with the visual tests document: [http://localhost:8124/programs/editor/src-localeditor.html#../../webodf/tests/visual-tests.fodt](http://localhost:8124/programs/editor/src-localeditor.html#../../webodf/tests/visual-tests.fodt)