Layers can be removed from MapFrames.
Introduced static Symbol factory function to AbstractSymbol.
Introduced IDs to ISymbol implementations (AbstractSymbol and subclasses).
removeLayer and removeSymbol method stubs.
SymbolLayer: Symbol access.
AbstractSymbol: hasPropertyName.
Implemented SymbolLayer Cleanup-Method. MapFrame listens for SpaceModels CHANGE Event.
Lazy initialisation of SpaceModelEntity.as
Cleanup.