Use "Editing" instead of Edition in doc

Summary:
"Edition mode" and "fast edition" don't make sense in English.
I renamed two images to match the improved language.

Reviewers: smankowski, yurchor

Reviewed By: yurchor

Subscribers: yurchor, kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D29837