Workspace KCM Code Improvement

Summary: Code formatting is improved. "save" function is improved by preventing unnecessary file open and file write. Functionality is same. Everything is tested, works fine.

Reviewers: ngraham, romangg, #plasma, mart

Reviewed By: romangg, #plasma

Subscribers: mart, davidedmundson, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12974