Make Akonadi compile without QT_STRICT_ITERATORS

Summary:
Without QT_STRICT_ITERATORS QVector and QList iterators do not satisfy the
STL iterator requirements since they are simply just T*. We can work around
that by introducing a helper to provide the necessary traits for those.

At the same time, turns out QT_STRICT_ITERATORS does not introduce any
overhead (they are inlined so everything gets optimized), confirmed by
comparing the produced assembly in Compiler Explorer.

Test Plan: Compiles

Reviewers: vkrause, dfaure

Reviewed By: dfaure

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20306