Create widget content only on first attachment

[spoenemann]

Fixes #42.

I also added an injection key DiagramWidgetFactory that can be used to create a custom DiagramWidget without the need to override createWidget in the WidgetManager (which is error-prone because you have to copy code from the superclass).

In case you have multiple diagram types with different DiagramWidget subclasses, you can use a child container to link each widget to its manager:

bind(MyDiagramManager).toDynamicValue(context => {
    const childContainer = new Container();
    childContainer.parent = context.container;
    childContainer.bind(DiagramWidgetFactory).toFactory(_ => {
        return (options: DiagramWidgetOptions, widgetId: string, diContainer: Container, connector?: TheiaSprottyConnector) =>
            new MyDiagramWidget(options, widgetId, diContainer, connector);
    });
    return childContainer.resolve(MyDiagramManager);
}).inSingletonScope();

[JanKoehnlein]

LGTM