Пакет apt-indicator: Информация

- Adapted to changed API in apt-0.5.15lorg2-alt72
  (pkgCacheFile class in RAII style).

- fix build with new Qt

- Just added some compiler flags (which don't change anything):
  + to adapt the C++ standard for the upcoming APT API (increased to C++17);
  + to be sure that APT API has not been used wrongly and will not be.
    (Namely, to be sure that if a method override was intended, it would
    actually be overriding and not hiding a virtual method and that it
    would be marked "override" for future.)