When the focused group changes, instead of reloading the whole
view just unload the currently displayed annotations and then
reload the annotations for the newly selected group(s) and
any drafts of unsaved annotations.

The only immediately 'visible' effect of the change is
that switching groups now only results in a call to /search
rather than network requests to fetch session state and features.

In future this should make it easier to preserve state
for the area below the top bar when switching groups.