Add support for computed node size in View definitions

[pcdavid]

Sirius Desktop supports NodeStyleDescription.getSizeComputationExpression() and we support it when interpreting VSMs (see WorkspaceImageDescriptionConverter.convert(), but not when defining diagrams directly using the View DSL.

The corresponding field should be added to org.eclipse.sirius.web.view.NodeStyle and used when creating the style provider for a node in ViewConverter.convert(NodeDescription, AQLInterpreter).