QWindow::setFullScreen() to use current screen instead of screen()

In virtual desktop environment call to QWindow::showFullScreen() will expand window onto screen returned by QWindow::screen() rather than current screen as QWindow is not aware it had been moved onto a different one.

A workaround with detecting current screen by iterating through QScreen::virtualSiblings() with assumption that top-left screen will be at [0] and bottom-right at [count()-1]; and calling setScreen before showFullScreen() has to be applied to get window shown on non-primary screen. This is rather annoying and there is no guarantee about screen order in QScreen::virtualSiblings().

This bug seems to be similar to https://bugreports.qt-project.org/browse/QTBUG-4365 , but is for QWindow rather than QWidget.

Also I reported connected suggestion: https://bugreports.qt-project.org/browse/QTBUG-29251