remove HSTRING instances

HSTRING needs to be released or handles will be leaked.
Instead use HString which takes care of resource management
on its own.

Task-Number: QTBUG-38115
Change-Id: I2c767776c1f22f45acd8dd77b693f30d63d894b9
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>