Port away from Qt's foreach: loop directly over pimpl member

Summary:
The old implementation of all the KActionCollection methods was a bit
inconsistent: some places loop directly over d->actions, while others do
it over the container copy created via call to the public getter method.
Without obvious reason for the different approach.

This patch ports the remaining foreach (and one ported before) to
a range-based for loop directly over the pimpl container object.

Reviewers: dfaure, cullmann

Reviewed By: cullmann

Subscribers: cullmann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23861