429642: [Model / CSS] Add properties view to define Model's stylesheets
https://bugs.eclipse.org/bugs/show_bug.cgi?id=429642

Signed-off-by: Mickael ADAM <mickael.adam@ALL4TEC.net>
Merge branch 'bugs/429642' of
https://github.com/Gabitchov/org.eclipse.papyrus.git into bugs/429642

Conflicts:
	plugins/infra/gmfdiag/css/org.eclipse.papyrus.infra.gmfdiag.css.properties/src/org/eclipse/papyrus/infra/gmfdiag/css/properties/databinding/AddModelStyleSheetCommand.java
	plugins/infra/gmfdiag/css/org.eclipse.papyrus.infra.gmfdiag.css.properties/src/org/eclipse/papyrus/infra/gmfdiag/css/properties/databinding/ModelStyleSheetObservableList.java
	plugins/infra/gmfdiag/css/org.eclipse.papyrus.infra.gmfdiag.css.properties/src/org/eclipse/papyrus/infra/gmfdiag/css/properties/databinding/RemoveAllModelStyleSheetValueCommand.java
	plugins/infra/gmfdiag/css/org.eclipse.papyrus.infra.gmfdiag.css.properties/src/org/eclipse/papyrus/infra/gmfdiag/css/properties/modelelement/CSSModelElement.java


Signed-off-by: Mickael ADAM <mickael.adam@ALL4TEC.net>