Fix or -Wno-error all warnings, fix building against Debian testing
- debian/: stop using dh_translations
- CMakeLists.txt: update C++ standard to C++ 14
- Wraps Telepathy-Qt's deprecation-causing headers in diagnostic pragma
- Fix QTimer::elapsed() deprecation by moving to QElapsedTimer
- Fix QString::null deprecation
- Fix QFlags' Zero constructor deprecation
- Fix qSort() deprecation by moving to std::sort()
- Fix qRand() deprecation by migrating to QRandomGenerator
- Enable -Wno-error for QVariant's comparative operators deprecations
- Revert "debian/rules: temporarily disable -Werror"