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