Allow use of XInclude in `IntelliSense.xml` files

I usually have several modules with functions in my AddIns which could also be used separately. To not make the `.IntelliSense.xml` file big and easier manageable, it would be great it would be possible to use [`XInclude`](https://en.wikipedia.org/wiki/XInclude). That would offer the possibility to write an own XML file for each module.

I already gave it a try but was unsuccessful to make it work. Thus I either do something wrong or XInclude is currently not supported. The main `.IntelliSense.xml` file looks like

```xml

```
and the "sub" `.xml` files look like
```xml

```