SystemDialog: use `Qt.WindowCloseButtonHint` flag to add close button to decoration

According to the source code in qxcbwindow.cpp, WM_TRANSIENT_FOR will
only be set when a window belongs to certain types. When focus stealing
prevention is enabled, KWin tries to match transient window, and it will
block window activation when WM_TRANSIENT_FOR is not set.

See also https://codereview.qt-project.org/c/qt/qtbase/+/448355

BUG: 462061
FIXED-IN: 5.27