Add ability to use bookmarks.

[duhrer]

Currently, bookmarks are not controllable using the gamepad navigator. We could trivially add an action to add a bookmark for the current page.

However, I don't see much point in doing this until we also add the ability to at least view and visit bookmarks using a modal of some kind. As bookmarks can be organised into nested groupings, this presents a more complex set of navigation challenges. you'd need to have a click to expand and contract group names, and clearly distinguish those.

Clicking a bookmark should open it, I'm thinking in a new tab by default.