Close tool tip instead of dialog if Escape is pressed

By default, QDialog rejects the dialog if Escape is pressed. If one of
the "Explain" tool tips of the CertifyWidget are shown, then pressing
Escape should just close the explicitly shown tool tip instead of
canceling the dialog. For simplicity, we hide any currently visible
tool tip on Escape.

• GnuPG-bug-id: T6046