When selecting an element in the browser's DOM inspector, we now also
show the resolved CSS properties (and their values) for that element.
Since the inspector was growing a bit more complex, I moved it out of
the "show inspector" action callback and into its own class.
In the future, we will probably want to migrate the inspector down to
LibHTML to make it accessible to other clients of the library, but for
now we can keep working on it inside Browser. :^)
