LibWebView: Add keyboard navigation to the Inspector

[circl-lastname]

Now keep a track of the DOM nodes which are actually visible in the Inspector, and allow the user to navigate the tree using the keyboard.

[circl-lastname]

Let me know if that's an okay caching solution

[AtkinsSJ]

Slightly tangential to your work, but I think our concept of a focused_element() might be wrong. The spec links from this step say:

Focusable areas can be elements, parts of elements, or other regions managed by the user agent.

Each focusable area has a DOM anchor, which is a Node object that represents the position of the focusable area in the DOM.

If I'm reading this right, it could be a different kind of Node than just an Element. This makes the verify_cast<Element>(candidate) below a bit dangerous. I think maybe we should instead have focused_node(), but I'm not entirely confident I'm understanding this right.

[circl-lastname]

#628 (comment)

I've addressed that here, the focused_element will as far as I know always be an element in the current implementation.
Specifically, you can see the piece of code I linked that prevents the focused_element from becoming the document, contrary to the spec, so I think it is safe to keep for now.

[AtkinsSJ]

To comment on actually trying this out: It's very nice! I'd like if the left and right arrow keys also worked to expand/contract elements and navigate to their parent or child, but that can go in a separate PR.

[ADKaster]

I think we'll end up addressing the "which node is really focused" stuff when we revisit this, so for now this is sweet! More DX for the Inspector is always appreciated