Refactoring of the graphical viewers to integrate better EMF Compare
framework

- AbstractGraphicalMergeViewer inherits from MergeViewer now. It is
almost the same code as StructuredMergeViewer but specialized for
diagrams with a specific getter of the GEF AbstractEditPartViewer.
StructuredMergeViewer and AbstractGraphicalMergeViewer could be merged
into a common class.
- DiagramContentMergeViewer inherits from EMFCompareContentMergeViewer
now. DiagramCompareContentMergeViewer does not exist anymore. Some
methods from EMFCompareContentMergeViewer have been fragmented to ease
this integration. 
- Improvements on the graphical viewers: no interactions anymore with
users on opened diagrams, the graphical selection is active only on
match selection.
- Delete of the contribution to
org.eclipse.gmf.runtime.diagram.ui.decoratorProviders and last code
referencing old decoration mechanism. 
- Happy checkstyle
- The whole code is internal now