Add support for additive merge

* Add a custom ResourceMappingMerger and specific mergers associated
(reference merger and conflict merger)
* Add a mechanism to select the merger for a diff not only by rank, but
also by another criterion, whose semantics is free (IMergeCriterion)

Change-Id: Ib32f8430bc58e69e627640efbafecdfae0279f04
Also-by: Mathieu Cartaud <mathieu.cartaud@obeo.fr>
Signed-off-by: Laurent Delaigue <laurent.delaigue@obeo.fr>
117 files changed