Need to fix warnings

[nonlin-lin-chaos-order-etc-etal]

src/mainwindow.cpp: In constructor ‘MainWindow::MainWindow(std::shared_ptr<std::basic_iostream<char> >, QWidget*)’:
src/mainwindow.cpp:121:28: warning: ‘QPalette::Background’ is deprecated: Use QPalette::Window instead [-Wdeprecated-declarations]
  121 |     pal.setColor(QPalette::Background, Qt::red);
      |                            ^~~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/qwidget.h:48,
                 from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/qmainwindow.h:44,
                 from /usr/include/x86_64-linux-gnu/qt5/QtWidgets/QMainWindow:1,
                 from src/mainwindow.h:7,
                 from src/mainwindow.cpp:1:
/usr/include/x86_64-linux-gnu/qt5/QtGui/qpalette.h:101:22: note: declared here
  101 |                      Background Q_DECL_ENUMERATOR_DEPRECATED_X("Use QPalette::Window instead") = Window
      |                      ^~~~~~~~~~
src/mainwindow.cpp:121:28: warning: ‘QPalette::Background’ is deprecated: Use QPalette::Window instead [-Wdeprecated-declarations]
  121 |     pal.setColor(QPalette::Background, Qt::red);
      |                            ^~~~~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtGui/qpalette.h:101:22: note: declared here
  101 |                      Background Q_DECL_ENUMERATOR_DEPRECATED_X("Use QPalette::Window instead") = Window
      |                      ^~~~~~~~~~
src/mainwindow.cpp: In member function ‘void MainWindow::iconActivated(QSystemTrayIcon::ActivationReason)’:
src/mainwindow.cpp:604:82: warning: ‘QTextStream& QTextStreamFunctions::endl(QTextStream&)’ is deprecated: Use Qt::endl [-Wdeprecated-declarations]
  604 |         qDebug() << "MainWindow::iconActivated(): unknown reason: " << reason << endl;
      |                                                                                  ^~~~
In file included from /usr/include/x86_64-linux-gnu/qt5/QtCore/qdebug.h:49,
                 from /usr/include/x86_64-linux-gnu/qt5/QtCore/QDebug:1,
                 from src/mainwindow.h:52:
/usr/include/x86_64-linux-gnu/qt5/QtCore/qtextstream.h:293:75: note: declared here
  293 | Q_CORE_EXPORT QT_DEPRECATED_VERSION_X(5, 15, "Use Qt::endl") QTextStream &endl(QTextStream &s);
      |