UML-282: Documentation Table

A new viewpoint is defined which extends the standard UML
representations. This viewpoint is named 'UML Extensions' and
provide a new table representation. This table contains two columns :
- Column 1 contains all the elements defined in a package
- Column 2 contains the comment associated to the UML element.
As a UML element can contain many UML comments, the table allows to
update only the first comment in the ownedComment list.
Define EEF properties for table viewpoint
UML-282 : Activate UML Extensions viewpoint

On UML project creation action, the newly created UML Extensions
viewpoint is activated by default as it was done before for the
structural and behavioral viewpoints.