`BucketBar` was setting the focus when hovering on the left-pointed
buckets. That relied on direct access to the anchors, which it is not
always possible. Instead, now the `BucketBar` communicates with a new
RPC event, `focusAnnotations`, in the `guest-host` channel (analogous to
the `focusAnnotations` in the `sidebar-guest` channel.

I have made a small improvement: when hovering a left-pointed bucket,
not only focus the anchor's highlights but also the corresponding
annotation cards in the sidebar. The same can be done for top and
bottom-pointed buckets.