[430527] Include the packed JARs in the p2 repo

According to https://wiki.eclipse.org/Tycho/Pack200, it looks like we
were simply missing the final step, i.e. actually include the packed
JARs into the p2 repo produced. Only the next build will allow us to
verify if this was really the issue.

Change-Id: Ibfdd2047485c982787db43ff70dd2387dbae7ba4
[431510] Avoid widget is disposed errors

The problem was in SectionDescriptor.getSectionClass(). This
SectionDescriptor stored the instance of ISection.

The standard SectionDescriptor
(org.eclipse.ui.internal.views.properties.tabbed.view) creates a new
instance each time the getSectionClass() is called.

In our case,
org.eclipse.sirius.eef.section.SectionDescriptor.getSectionClass()
returned a ISection with disposed controls. This explains the "Widget is
disposed".

Bug: 431510
Change-Id: I50d2ed42c437a653bd15c0b89d766851bd15ee87
Signed-off-by: Maxime PORHEL <maxime.porhel@obeo.fr>