undo: Port away from foreach loops over members without calls to owner

Summary:
There is some small risk here:

• overseen call chains which still call the owner and modify the container
• other threads might access the same containers, even if class is not designed to be thread-safe, but wrong usages are currently caught mostly by the container copy

GIT_SILENT

Reviewers: Kate, dhaumann

Reviewed By: Kate, dhaumann

Subscribers: dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23701