If a container is given to mapReduce, it takes the responsibility to
report progress information for the whole operation. If the map function
reports its own progress, that is taken into account for the overall
progress.

The (so far only unordered) Utils::map operation can be used to replace
MultiTask, by passing a member function of the items in the container as
a map function.

Change-Id: I18ca38a6ad2899d73f590bfe59bf2e6eb2f1a57a
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>