Use a `MutationObserver` to update the tooltip. This allows us to update
the tooltip after the page and user script have fully loaded, and then
the label gets populated. It also should allow us to capture updates to
the label and update the tooltip accordingly.

The script was running too early, and the content of the element wasn't
populated before it tried to set the title. Try running at
`document-idle` to see if that fixes it. It doesn't. Looks like we're
going to have to use a mutation observer. We would have needed to
anyway, though, because the text can change while the page is open, and
we want the tooltip to update accordingly.