Only consider fullscreen elements' children for link hints

This PR stops us from showing link hints for elements concealed by fullscreening when an element is made fullscreen.

Fullscreen youtube videos are the best example of this; it's useful to be able to click the play/pause and fullscreen buttons, but we also show a lot of link hints for links on the underlying page, which we can't see.
Searches from the `contextNode` provided to `evaluateXPath` rather than always from the document root.