qtquick: Use the fallback mouse grabber

This code was needing porting.

src/qtwidgets/Platform_qtwidgets.cpp

@@ -124,3 +124,9 @@ View *Platform_qtwidgets::createView(View *parent) const
 {
     return new Views::View_qtwidgets<QWidget>(nullptr, Type::None, Views::View_qtwidgets<QWidget>::asQWidget(parent));
 }
+
+bool Platform_qtwidgets::usesFallbackMouseGrabber() const
+{
+    // For QtWidgets we just use QWidget::grabMouse()
+    return false;
+}