Implicitly shared class template: improve method declarations

Summary:

• Create special definitions for
  • default constructor
  • copy constructor
  • assign operator
• Use common macros for method signature lines instead of own copies
• Add definitions for any {public,protected,private}_functions

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5709