qt4: Replace the custom, secure entry widget with the standard widget.
* configure.ac: Remove enable-pinentry-qt4-clipboard option. * qt4/qsecurelineedit.cpp, qt4/qsecurelineedit.h, qt4/qsecurelineedit_p.h, qt4/secstring.cpp, qt4/secstring.h: Removed. * qt4/Makefile.am: Update accordingly. * qt4/main.cpp (qt_cmd_handler): Use QString instead of secqstring. * qt4/pinentrydialog.cpp (PinentryDialog::PinentryDialog), (PinEntryDialog::error, PinEntryDialog::pin), (PinEntryDialog::updateQuality): Use QLineEdit and QString instead of secmem variants. * qt4/pinentrydialog.cpp (PinentryDialog::PinentryDialog): Set echo mode to password. * qt4/pinentrydialog.h: Update accordingly.