* qt/main.cpp (qt_cmd_handler): Set text format of message box to
PlainText. Set text interaction flags of message box to
TextSelectableByMouse.
* qt/qt/pinentryconfirm.h (class PinentryConfirm): Derive from
QAccessible::ActivationObserver interface; add d'tor; override
focusNextPrevChild method of QMessageBox and  accessibilityActiveChanged
method of QAccessible::ActivationObserver interface.
* qt/pinentryconfirm.cpp (messageBoxLabel): New.
(PinentryConfirm::PinentryConfirm): Register as activation observer.
(PinentryConfirm::~PinentryConfirm): New.
(PinentryConfirm::focusNextPrevChild): New.
(PinentryConfirm::accessibilityActiveChanged): New.