AbstractBrowsingContext has a subclass RemoteBrowsingContext without a
visit_edges() override (and it doesn't really need one). But currently,
we rely on subclasses visiting AbstractBrowsingContext's opener BC.
This adds a visit_edges() to AbstractBrowsingContext to explicitly visit
the opener BC itself.
Support for this element has been removed from all major engines years
ago already, and it's currently the only reason we have a weird
"visible" flag on Layout::Node (which we toggle on a timer here..)
This event is the star of the show, and the main way that web content
can react to either programmatic or user-initiated navigation.
All of the fun algorithms will have to come later though.
This API is how JavaScript can manipulate the new Navigable concepts
directly. We are still missing most of the interesting algorithms on
Navigation that do the actual navigation steps, and call into the
currently WIP navigable AOs.
