root: show "Loading..." while listing containers (fixes #48)

As for listing objects in a container, this is mostly useful when
authentication is needed when listing the containers.
rootCtrl: call listObjects until it returns less than limit

Before we would call listObjects again as long as we get anything at
all back from Swift. This lead to an extra unnecessary query: we know
that we'll get zero objects back when the last query returned less
than limit objects.
e2e: use more specific selectors

Instead of selecting table elements by nth-child, we now try to select
them using the binding instead. This should be more stable if columns
are added or removed in the future.