Improve accessibility of Diagnostics/Show Audit Log label

* Add UrlLabel, a label for displaying a single link, which
  * automatically gives focus to/selects the displayed link when the
    label gets focus,
  * sends a text selection event to the accessibility clients when
    a link is selected,
  * works around a bug in QLabel::focusNextPrevChild(false)
* Add AccessibleRichTextLabel which allows accessibility clients to
  retrieve the selected text of a UrlLabel
* Use UrlLabel for the Diagnostics/Show Audit Log label

GnuPG-bug-id: 5535