Adjust binding for CommandActionHandlerInitializer (#473)

Make is easier to customize/rebind the
`CommmandActionHandlerInitializer` by binding to itself before binding
it as IActionHandlerInitializer.

(All other multiinjected keys already follow this pattern. See
#239)

packages/sprotty/src/base/di.config.ts

@@ -71,7 +71,8 @@ const defaultContainerModule = new ContainerModule((bind, _unbind, isBound) => {
     bind(TYPES.IDiagramLocker).to(DefaultDiagramLocker).inSingletonScope();
 
     // Action handler
-    bind(TYPES.IActionHandlerInitializer).to(CommandActionHandlerInitializer);
+    bind(CommandActionHandlerInitializer).toSelf().inSingletonScope();
+    bind(TYPES.IActionHandlerInitializer).toService(CommandActionHandlerInitializer);
 
     // Command Stack ---------------------------------------------
     bind(TYPES.ICommandStack).to(CommandStack).inSingletonScope();