Purge the plugin.xml with old E3 extension points
Put all initializations of the Rental UI 'feature' in the RentalAddon,
and namely the PreferenceStore instance. The RentalUIActivator is no
more used for initialization because injection can not be used at this
level. Only Addons provide injection mechanism. Unfortunately I had to
create a static accessor method in RentalAddon to use the
PreferenceStore in the classes that are not instanciated using
ContextInjectionFactory (prefinit and NonePalette)... I will change it
later...
Last point : the preferenceStore for Rental can be injected using the
@Named(RentalUIConstants.RENTAL_UI_PREF_STORE) name.