Move Settings Editor example into manual tests

As an example it is outdated and not showing best practices. It would have been a good opportunity to show QTreeView with a custom item model for non-trivial data, but instead it uses slow and clunky QTreeWidget.

The screen shot (shows "Trolltech") and example details (use "QSA", aka "Qt Script for Applications" settings) are quite outdated.

Almost all of the example is about widget layouts and item views (for which we have better examples), and not about QSettings.