Performance improvements for coverage

Usage of QUrl is replaced with QString, since QUrl has very poor performance. Furthermore, a QSource object is introduced, representing a unique source document. Over time, we should replace all usage of hashes and urls to identify source locations with a getID call on the QSource object (or a QCodeBlock object).
Merge branch 'master' of github.com:cs-au-dk/Artemis

Conflicts:
	WebKit/Source/WebKit/qt/Api/qwebexecutionlistener.cpp
	WebKit/Source/WebKit/qt/Api/qwebexecutionlistener.h
	artemis-code/src/model/coverage/coveragelistener.cpp
	artemis-code/src/model/coverage/coveragelistener.h
	artemis-code/src/model/pathtracer.cpp
	artemis-code/src/model/pathtracer.h
	artemis-code/src/runtime/browser/webkitexecutor.cpp
Merged