Add new constructor for KDevelop::DetectedProblem class

Summary: The patch adds new constructor for KDevelop::DetectedProblem which allows to avoid creating of subclasses with only overloaded source() and sourceString() methods for "problem-checker" plugins like cppcheck/valgrind.

Test Plan: Tested with master branch and cppcheck/valgrind plugins

Reviewers: mwolff, kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kfunk, mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4883