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