Add pinentry-qt4-clipboard option
Enabling this option will make it possible to paste a passphrase into pinentry-qt4. This defeats the secmem mechanism but drastically increases usability for some users. * configure.ac: New option pinentry-qt4-clipboard. * qt4/qsecurelineedit.cpp, qt4/qsecurelineedit.h: Activate clipboard and context menu if PINENTRY_QT4_CLIPBOARD is defined.