[INTERNAL] Add PreferenceStore-related Eclipse-look-alikes to Saros/I

This contains two interfaces and one class (actually more of a data
type).

Change-Id: Ice3b6502faa99a8554f05b5be7a17f22755af29e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1584
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>