[560861] Swapping content merge viewer don't swap phantoms The target side when revealing phantom decorators don't return the correct MergeViewerSide when the compare configuration is mirrored. - Fix the method PhantomManager#getTargetSide(Match, View) - Refactor DiagramContentMergeViewer class - Creating Unit tests Bug: 560861 Change-Id: I437293113ad2e5fe91ffb4977aaa52c975167d73 Signed-off-by: Glenn Plouhinec <glenn.plouhinec@obeo.fr>