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