Add keyboard shortcuts to select/expand/collapse elements in element-trees

[sachinhosmani]

Just like in inspector, where the up/down/left/right buttons change the selected element and allow expansion/collapse of elements, we could add key bindings in the tool too.

[addyosmani]

If you add key-bindings in the extension, it would be useful to figure out some UI for letting users know what shortcuts are supported. In DevTools we have (ironically) a shortcut for this, but also a summary pane. Might be worth including.