add eReference reference on UMLDiff EClass
new UML cases
added cross reference differences on Comparison#getDifferences(EObject)
filter out diff refined by or required by refines by
add specific TextMergeViewer to remove support for partial merge
change how groups are handled: no more adaptation to EDiffNode now that
we have our custom content provider
add specific mehtods for content providers
add new uml2.ide.ui plugin to specialize UI
add uml2.ide.ui
Change isThreeWay to an attribute

isThreeWay used to be a method browsing the comparison model to see if
some origin has been defined in some ResourceMatch. This could only work
for comparisons having resources and not plain EObjects comparison.

Keeping the same approach would have meant to browse through the whole
comparison model to see if some Match had a reference to an origin. That
looks like quite a bad idea.

So, this commit changes the approach, now the match engine itself set an
ttribute to specify whether its three way or not. Checking for it is now
only an attribute access.
Add predicates for Real and Pseudo conflicts.
Avoid resolution of feature values from the Diff engine
Prevent the UI from resolving references when displaying Diffs
fix clipping area issue on windows
use client area instead of bounds of the Table
add lines between diffs
add cases
setThreeWay should be call as early as possible (test failure before
this commit)
some refactoring to make the code clearer
change DiffCrossReferencer to handle subclasses of Diff.