Add ClangClassHelper, to restore features of CppClassHelper

Summary:
Based on files languages/cpp/codegen/cppclasshelper.*
as removed by f31d32f49458897c97388a4b8f40f97e5af02f59

Needs "C++" added to values of X-KDevelop-Language for kdevclangsupport.json,
so kdevclang plugin is found when a "C++" language plugin is queried.

Needs someone with clue to complete whatever is done for the variables
passed via the "included_files" key.

Test Plan:
Generation via File template dialog provides default methods
for a C++ class again and C++ templates have content again
if special variables for groups of private, protected and public
methods & members are used.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, arrowdodger, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4359