Add keyboard shortcuts for [ and ]

These keyboard shortcuts operate on the focusable element concept by
focusing either the previous or next element, respectively, immediately
followed by "opening" the focused element. This enables efficient
navigation between sets of elements where you want to see the detailed
page for each element, such as when reviewing snapshots in a sweep.

Change-Id: I7e60381a74e46f96d58db5bfff78e82690c0407f