Implement QApplication::topLevelAt() using QGuiApplication::topLevelAt().

Reducing code duplication, preparing for High-DPI support.

Task-number: QTBUG-38858
Change-Id: Id0ed12db4b5cadd0eef79afd0ac62f58a7b01901
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>