[434099] remove usages of SiriusUtil.isFromSirius

SiriusUtil.isFromSirius is checking for the class namespace
to return true. This has strong implication regarding extensibility
as any code using this predicate will filter subclasses or sub
interfaces of Sirius which are not in the org.eclipse.sirius namespace.

This commit replace the internal usages of isFromSirius() by instanceof checks
which will return true for all the subtypes.
It introduces a "marker" Interface ISiriusEditPart to do so.

Change-Id: I82878400f78234b03ce8733c98439f830ee1aff3
[427273] Improve layout for the default sized help window

Change-Id: Ic6dbf6d1ca2f763db08a65a1329e48bf79fd5dde
Signed-off-by: Maxime PORHEL <maxime.porhel@obeo.fr>
[427278] Make each feature is branded separately

Bug: 427278
Change-Id: I27cfdb2ae0a05ce6c237c8b61680d03e24ab59dd
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>