designer/qmltextgenerator: get rid of deprecated QVariant::Type

removes warning due to QMetaType::Float->QVariant::Type conversion.

Task-number: QTCREATORBUG-12237
Change-Id: I428d4f285005da3befe55626c1f04a3beaa8ac33
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>