[432986] Enable EMF's child extender mechanism

When moving the diagram metamodel out of the core, we lost some creation
items in the VSM editor's context menus, as they moved into an EPackage
the core genmodel does not know about. Sirius already provides several
ad hoc mechanisms to handle this kind of things (see
DialectUISercice.provide*() methods), but none of them correspond to the
locations that need to be extended in this case.

Instead or creating yet another ad hoc extension method (or two), enable
EMF's native mechanism to handle this. It should probably be enabled
everywhere and replace our custom mechanisms, but for now just do the
minimum:
* enable "Extensible Provider Factory" in all the description EPackages
  in viewpoint.genmodel so that the item providers look for extensions;
* enable "Child Creation Extenders" in all the description EPackages in
  diagram.genmodel so that the extensions for viewpoint.ecore are
  properly generated;
* remove the custom contributions which were added in
  DiagramDialectUIServices.provideNewChildDescriptors(), as they are now
  redundant and create duplicate entries in the menus.

The patch is rather large, but the only things manually changed are a
few boolean in the 2 genmodels and the change in
DiagramDialectUIServices.provideNewChildDescriptors(). All the rest is
the result of the EMF re-generation.

With this change, we get back the 4 menu items which were identified as
missing compared to M6 (before the split). See the bugzilla for the
reasons why we think they are the only ones.

Bug: 432986
Change-Id: I5735dc63f75c7292af1ee7640fc2a39399ea293d
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[431359] Move the definition and implementation of aird resources into the core plug-in

For historical reasons the session resources (*.aird) depend on GMF and
are defined in the plug-ins for the diagram dialect. However some of the
code in the core (not diagram-specific) plug-in needs to load these
models (at least to correctly identify them as session resources). This
fails if the core plug-ins are installed but not the diagram-specific
ones.

This can happen rather easily since for example a Viewpoint
Specification Project created with our own wizard depends on
org.eclipse.sirius but not the diagram part. If a user installs such a
plug-in, p2 will only get the required core plug-ins, not the whole
feature with the diagram-specific parts.

Move all the required classes into the org.eclipse.sirius core plug-in.
It makes that plug-in depend on parts of GMF runtime, at least until we
can properly remove all the dependencies towards GMF, but that is less
severe than exposing users to potential crashes.

Bug: 431359
Change-Id: I4bdaea9c2e387c13549b845abaf0ba93a2d7de64
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>