Listen for mouseup/mousedown events on the document body rather than document
itself, so that we can still capture them if an event listener on
`document.body` or `document.documentElement` stops propagation of the event.
The VitalSource integration does this for example to prevent VitalSource's own
selection UI appearing, but ordinary web pages could do something similar.

In VitalSource books, this change makes the adder behave the same as it does in
HTML and PDF documents where, when making a selection with the mouse, the adder
only appears when the mouse is released so it doesn't get in the way during the
selection.

While updating the tests for this change, they were converted to use a real
document, as the complexity of mocking the relevant parts of the DOM document
interface accurately is now too high.