Improved highlight movement ++

[midsbie]

1. User can move through set of highlights if there are 2 or more highlights which are visible on the page
2. Highlight movement should consist of: (i) ordering highlights by position on page, which may not necessarily coincide with position in the DOM; and (ii) ignoring invisible highlights by inspecting bounding box and testing for 0 height, which is believed should catch majority of cases

(1) above simply requires the test outlined in (1.ii) to be conducted for two highlights.

Implementing this issue should greatly improve the UX of highlight movement.

[midsbie]

3. Also center horizontally if out of bounds