Eglfs: implement screen capturing for qml

Screen capturing of qml eglfs application needs a separate implementation.

QQuickWindow::grabWindow should work for this.

A possible way to implement:

• Get existing windows via QApplication::topLevelWindows.
• Find the needed window by qobject_cast<QQuickWindow*> and QWindow::screen == screen.
• Check if QQuickWindow::works in a separate thread. If so, create a grabber with the matching threading flag.
• Use window->grabWindow in the grabber.