perist recent projects via PreferencesService

As the approach used before (IMemento in ApplicationWorkbenchAdvisor)
seems to be no longer supported by Eclipse.

Change-Id: Ibbb651649c8484f78aa5b836bfce2e48730a27ed
Reviewed-on: http://gerrit.esdi-community.eu/3133
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>