Renderer.add should allow us to inject a component at the specified index

Currently it is not possible to inject a component at a specific index. It is just pushed to the end in the given slot.

```javascript
this.pluginStore.executeFunction("Renderer.add", "header-right", () => Hello world!);
```

While we can achieve the required order by changing the order of installation of plugins, it is not the best way because the order of installation should be independent of the order of components in UI.

We need to be able to specify an index with this method.