15 oldest fixes pickuped from dev branch

• https://codereview.qt-project.org/c/qt/qtbase/+/310451
• https://codereview.qt-project.org/c/qt/qtbase/+/309673
• https://codereview.qt-project.org/c/qt/qtbase/+/306085
• https://codereview.qt-project.org/c/qt/qtbase/+/305023
• https://codereview.qt-project.org/c/qt/qtbase/+/303918
• https://codereview.qt-project.org/c/qt/qtbase/+/301333
• https://codereview.qt-project.org/c/qt/qtbase/+/300890
• https://codereview.qt-project.org/c/qt/qtbase/+/300559
• https://codereview.qt-project.org/c/qt/qtbase/+/297764 (fixed merge conflict)
• https://codereview.qt-project.org/c/qt/qtbase/+/282590 (fixed merge conflict, in dev xinerama support was droped)
• https://codereview.qt-project.org/c/qt/qtbase/+/278514
• https://codereview.qt-project.org/c/qt/qtbase/+/310966 (fixed merge conflict constexpr vs QT_SIZEPOLICY_CONSTEXPR)
• https://codereview.qt-project.org/c/qt/qtbase/+/272589
• https://codereview.qt-project.org/c/qt/qtbase/+/251578
• https://codereview.qt-project.org/c/qt/qtbase/+/235138 (fixed merge conflict)