Use KConfigDialogManager to manage main window settings
Summary:
They are still instant apply. But are now automatically saved again. This makes
the settings consistent when opneing a new instance when another is still
running.
Reviewers: ngraham, ervin, Spectacle
Reviewed By: ngraham, Spectacle
Tags: Spectacle
Differential Revision: https://phabricator.kde.org/D27415