[server] improve server API

[JanKoehnlein]

• add default type in SModelElements classes, e.g. node in SNode
• add a constant for empty model
• allow to add style classes
• ITraceProvider.trace() should retrun the SModelElement
• add a template method to handle additional actions in LanguageAwareDiagramServer#accept
• allow to configure ActionTypeAdapter using DI (don't use static methods) to add more actions
• IDiagramGenerator should take the Context as an argument
• IDiagramGenerator should take the Issues as an argument
• add a ServerStatus.Severity FATAL that means avoid diagram generation. ERROR could mean we can generate the diagram anyway, e.g. when there are only validation errors
• add SIssueMarker to model