Re-introducing pre-QRandomGenerator code

Commit d6258827c872fc5b7862d94 introduced QRandomGenerator which became
available with Qt 5.10. This Qt version is available in virtually all
Desktop Linux distributions (even Debian stable uses 5.11).
However, SailfishOS still uses Qt 5.6.3 at the time of writing. To make
code compile, #ifdef gates have be introduced, providing alternative
code paths for pre-5.10 environments.