Summary:
Until now every FolderView applet stored exactly the same screenMapping info in the config file. Store it only once
using the config object of the Plasma::Corona

Reviewers: #plasma, mwolff, hein, broulik, mlaurent, mart

Reviewed By: #plasma, mlaurent, mart

Subscribers: mart, mlaurent, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9300