Highlight API: way to select a highlight / edit the range of a highlight with native selection control

[hober]

In e-readers that have persistent highlight features, it's common to be able to tap on a highlight to edit it. When tapped, the highlight becomes a selection instead, and the selection's range can be tweaked just like you usually do, with the platform's native selection editing mechanism. De-selecting "commits" the change to the highlight.

Maybe this should be specified to be the default behavior when a highlight is tapped (see #54), and sites can do something else by calling preventDefault()?

[hober]

See also whatwg/dom#772