qt: Improve width of pinentryconfirm

* qt/pinentryconfirm.cpp (PinentryConfirm::showEvent): Add spacer
item for text width.

This fixes a pinentry-qt4 bug where part of the text might have
been hidden. And improves the layout for pinentry-qt5 where
the fingerprint will no longer be wordwrapped. Needs to be
done in the show event because only there we have the icon available.