Fix QT compile-time warnings

[flipmcf]

Fixes this compile-time warning:

./rddialog.h:33:65: warning: ‘constexpr QFlags<T>::QFlags(QFlags<T>::Zero) [with Enum = Qt::WindowType; QFlags<T>::Zero = int QFlags<Qt::WindowType>::Private::*]’ is deprecated: Use default constructor instead [-Wdeprecated-declarations]
   33 |   RDDialog(RDConfig *config,QWidget *parent=0,Qt::WindowFlags f=0);
      |                                                                 ^
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qglobal.h:1299,
                 from /usr/include/x86_64-linux-gnu/qt5/QtGui/qtguiglobal.h:43,
                 from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/qtwidgetsglobal.h:43,
                 from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/qapplication.h:43,
                 from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/QApplication:1,
                 from rdcart_dialog.cpp:24:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qflags.h:123:80: note: declared here
  123 |     QT_DEPRECATED_X("Use default constructor instead") Q_DECL_CONSTEXPR inline QFlags(Zero) noexcept : i(0) {}

[flipmcf]

Fixes #992

[flipmcf]

There's more I want to add... standby

[ElvishArtisan]

Merged to 'v4'. Thank you!