Listen for key events for keyboard shortcuts on the document element rather than
body element, to make them work in XHTML documents.

When no interactive element (eg. an input field) is focused in an HTML document,
keyboard events are sent to the body element. In an XHTML document however
keyboard events are sent to the document element instead in Safari and Chrome
(in Firefox they are still sent to the body). The key event listeners used for
the adder's shortcuts were installed on the body element, so they didn't work in
XHTML documents in Safari and Chrome.

My guess is that the reason for this behavioral quirk is that HTML documents are
guaranteed to have a body element - an empty one will be generated if none
is present in the markup, whereas XHTML documents are not.

Fixes https://github.com/hypothesis/client/issues/4364