Introduce XStream

newest version of XStream replaces XMLSerializer and XMLParser.
Validationtests currently disabled - ticket on whiteboard.

successful jenkinsrun: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/353/

run of failing testcase: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/246/testReport/org.catrobat.catroid.uitest.ui/MyProjectsActivityTest/