jalv.qt5 setCentralWidget#1

#include <QWindow>
#include <QWidget>
#include <QVBoxLayout>

    // QWidget *widget -- as returned from suil_get_widget().
    //
    QWidget *central_widget = new QWidget(this);
    QWindow *window = QWindow::fromWinId(widget->windId());
    QWidget *container = QWidget::createWindowContainer(window, central_widget);
    QVBoxLayout *layout = new QVBoxLayout();
    layout->setMargin(0);
    layout->setSpacing(0);
    layout->addWidget(container);
    central_widget->setLayout(layout);
    setCentralWidget(central_widget);