Don't load the KSharedConfig instance on every read
Summary:
At the moment it was loading >2000 times at plasma boot for me because some
settings are being used in AppletProxyModel::filterAcceptsRow.
Test Plan: Added a debug statement in KSharedPointer::openFile()
Reviewers: Plasma, jgrulich
Reviewed By: jgrulich
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D20522