[Pharo-project] Issue 1558: The old preference browser should be as much functionnal as possible while migrating to the new setting system

SLICE-PreferencesMigration-Part6 in InBox ----------------- make the old preference browser fully functional again. * hacked Preference>>preferenceValue, Preference >>preferenceValue:, Preference>>rawValue: and Preference>>togglePreferenceValue. * new Preferences class method category "settings-migration": For each preference migration, it contains 2 methods for getting and setting the preference. They are implemented using the new setting system. As an example for the migration of #annotationPanes: Preferences class>>annotationPanesSetting and Preferences class>>annotationPanesSetting: are added. ----------------- Also useful to understand correspondences between old and new preference systems