Fix subscript and superscript for QQuickText

Task-number: QTBUG-32948
Change-Id: I4920015b2604afbd9e2050f0225de98d5188d589
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>