Make the Tycho tests happy

The Tycho tests were failling because the tests themselves are
doing illegal instantiations of instances using a constructor
which is not visible.
In particular : LayerEntry and GridCell

The tricky part is :
- We don't want to expose the constructors
- We don't want to break the APIs

This commit :
- split the offending classes in an abstract and implementation one
- introduce internal packages for isolating the implementation one
- enables PDE tooling
Add about.html
Add an Example tab in the gallery for Paperclips