Set key/group summary if we have a key/group on editing finished

This has the effect that, when the user hits Enter after entering some
text matching exactly one key or group, then the text in the line edit
is "auto-completed" with the key/group summary text. Apart from the
visual feedback this should also trigger audible feedback by screen
readers. (ocra does read the auto-completed summary text.)

GnuPG-bug-id: 5845