This adds "Inspect Element" (currently the only entry) to the context
menu for the page, which will do what you expect (most of the time),
and bring up the Inspector with hovered element selected.
This has been broken since the switch to the multiprocess architecture
(and even before then was very limited).
This restores the previous functionally and also implements the ability
to inspect individual elements (by selecting them in the tree view).
The inspector also now correctly updates when navigating between pages.
