Refactoring: replace custom reference counting by std::shared_ptr

- For the template engine the following changes were made:
- TemplateVariant now stores a std::shared_ptr for TemplateListIntf and
  TemplateStructIntf instead of a raw pointer.
- In context.cpp SharedPtr has been replaced by std::shared_ptr
- Instead of Context object all implementing RefCountedContext now
  std::shared_ptr's are used to manange the livetime of objects;
  TemplateListIntfPtr and TemplateStructIntfPtr
- Internally two generic implementations are used TemplateList and
  TemplateStruct, also these are managed via std::shared_ptr.
- To avoid cyclic references; the TemplateVariant type can also
  hold weak references to TemplateStructIntf and TemplateListIntf.
- Context class alloc() functions now return either a
  TemplateListIntfPtr or a TemplateStructIntfPtr for more uniform
  handling of the objects.

Author: doxygen