Improve cursor positioning of certificate input field

QLineEdit moves the cursor to the end when a text is set. This has the
effect that the start of a longer key/group summary is not visible when
we set this summary. This change improves the cursor positioning:

• Keep the cursor position when a key/group is updated in the background.
• Move the cursor to the start when the user finishes editing (with

Return), selects a key/group via the selection dialog, or moves focus to
another UI element.

• Otherwise, we keep the default behavior of QLineEdit.

• GnuPG-bug-id: T6083