[XmlExtractor] Use QXmlStreamReader for better performance

Summary:
At least in the benchmark, QXmlStreamReader is more than twice as fast
as QDomDocument.

Test Plan:
Before/after:
RESULT : XmlExtractorTests::benchMarkXmlExtractor():

810 msecs per iteration (total: 810, iterations: 1)

RESULT : XmlExtractorTests::benchMarkXmlExtractor():

288 msecs per iteration (total: 288, iterations: 1)

Reviewers: Frameworks, astippich, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16591