Cleanup and modernize the Calendar library codebase

Summary:
* enable C++14
* use QScopedPointer for d-pointers
* remove dead/unused code
* refactor duplicated code into functions
* use named constants instead of string literals to avoid typos
  but also to reduce the binary size due to string duplication
* const'ify and auto'ify

Test Plan: Compiles, tests pass

Reviewers: barchiesi

Reviewed By: barchiesi

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D24085