Rework GmodelState binding

Bind actual implementation class of GModelState to itself as singleton before binding it to GModelState.
e.g. by overriding the following method of `GLSPDiagramModule`

```java
bindGModelState(){
  return MyModelState.class
}
```

clients can also directly inject the implementing subclass with
```java
@Inject 
protected MyModelState myModelState;
```

Also fix type in `GLSPServerModule`

plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/DiagramModule.java

@@ -166,7 +166,7 @@ protected void configureBase() {
       bind(DiagramConfiguration.class).to(bindDiagramConfiguration()).in(DiagramGlobalSingleton.class);
       bind(ServerConfigurationContribution.class).to(bindServerConfigurationContribution()).in(Singleton.class);
       // Model-related bindings
-      bind(GModelState.class).to(bindGModelState()).in(Singleton.class);
+      configureGModelState(bindGModelState());
       bind(ModelSourceLoader.class).to(bindSourceModelLoader());
       bind(GModelFactory.class).to(bindGModelFactory());
       bindOptionally(ModelSourceWatcher.class, bindModelSourceWatcher())
@@ -225,6 +225,11 @@ protected Class<? extends ServerConfigurationContribution> bindServerConfigurati
       return DefaultServerConfigurationContribution.class;
    }
 
+   protected void configureGModelState(final Class<? extends GModelState> gmodelStateClass) {
+      bind(gmodelStateClass).in(Singleton.class);
+      bind(GModelState.class).to(gmodelStateClass);
+   }
+
    protected Class<? extends GModelState> bindGModelState() {
       return DefaultGModelState.class;
    }

plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/ServerModule.java

@@ -86,7 +86,7 @@ public class ServerModule extends GLSPModule {
    public ServerModule configureDiagramModule(final DiagramModule diagramModule, final Module... mixinModules) {
       String diagramType = diagramModule.getDiagramType();
       Preconditions.checkState(!diagramModules.containsKey(diagramType),
-         "A module configuration is alreay present for diagram type: " + diagramType);
+         "A module configuration is already present for diagram type: " + diagramType);
 
       Module combinedModule = ModuleUtil.mixin(diagramModule, mixinModules);
       diagramModules.put(diagramType, combinedModule);