Don't pad beyond maximal width

Don't pad beyond maximal width

The old implementation behaved as if the "description" is the only
information that is being displayed, however in actuality the
available width has to be shared with the key (and the separator
between the two).

Failing to take that into account meant that every binding whose
description isn't *by itself* wider than the width available to
display all the information got excessive padding, which later has to
be removed during truncation again (resulting in misleading ellipses).