Set the menu role for the QPlatformMenuItem to the default

QAction has a default menu role which is passed on to the
QPlatformMenuItem. But as QtQuickControls does not use QAction, we need
to set the default menu role explicitly to TextHeuristicRole. This fixes
the merging of menu items on Mac.

Task-number: QTBUG-31883
Change-Id: I24f0e265f8ceca376db9970d62e8f79a646fef3b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>