Templating for SplitButton

```
[x ] feature request 
```
**What is the motivation / use case for changing the behavior?**
Please add a default slot to the main button (not the dropdown) in the SplitButton. This should be very similar to what is done in the Button component...

Would be good if we could also have access to the nested slot for the menu items (this is to get around #1627) ...