Announce current item when certificate list gets focus

QTreeView sends the accessible focus event for the current item (resp.
table cell) before the accessible focus event for the view itself when
the view gets focus. Screen readers (e.g. orca) ignore the first focus
event in favor of the later focus event and do not announce the focussed
table cell. Therefore, we force QTreeView to send another focus event
for the table cell by unsetting and re-setting the current index when the
view gets focus.

GnuPG-bug-id: 6095