Fix offset calculation in hover box.

[All-less]

Currently, the offset of a hover box is calculated based on offsetLeft and offsetTop. However, both attributes are relative to the parent, so the citations and footnotes in a table (or other nested elements) may be placed incorrectly.

This patch calculates the offset by recursively adding up the offsets of parents. Now the hover boxes appear at the right place.

[colah]

Gosh! Thank you for the lovely PR. GIFs in the description is lovely.

I'll defer to @ludwigschubert on code review, just wanted to comment because you'd handled that so nicely. :)