Other modules should be able to use the diff utility by calling the API. A diff
provider (a service, that retrieves the file differences) and diff presenter
(the visual display of these differences) can be defined to be able to plug
together an arbitrary diff provider and diff presenter. The module
implementation should then define one diff provider (currently it's an external
utility, in the future it should be Java native diff) and one diff visual
presenter (it will be moved from vcscore module).
Confirming in development build #200206190100 of NetBeans 3.4.