Revert renaming on several UI Strings

I went through the plugin.properties files looking for
wrong usage of the name Sirius versus Viewpoint

Most visible ones being the Wizard descriptors, fixed
with this commit.
Make Custom Properties displayed again in VP Editor

Some of the customized properties views were not displayed since
the re-location of the viewpoint EPackage into sirius.viewpoint.

Some plugin.xml were referring to non existing types
  org.eclipse.sirius.description.SomeType

This commit changes them in
  org.eclipse.sirius.viewpoint.description.SomeType.

It also revert back changes made during the re-generation
regarding the IdentifiedElement property sections : I reapplied
the strategy used originally (before the renaming) but this doesn't
look robust regarding re-generation. One has to re-apply this change
each time.

The change is : Disable the Label property section generated for
the IdentifiedElement EClass, and make sure the Name property
section in the plugin.xml is using the *Spec class which got
customized

Change-Id: Ife20db24df7982258d9efe20b2edfddfb6e010b6
Re-Add the Viewpoint icon

This icon is displayed (at least) in the Select Viewpoint action
(contextual to a modeling project or to an .aird file, depending
on the case)

It got replaced with a generic EMF diamond, this commit add the
original icon back.

Change-Id: I5b1edbdfd1e381bc5ccd95c26f8f40670b0fdb72