SplitButton: add the possibility to pass properties to the TieredMenu component

[RolT]

SplitButton has buttonProps & menuButtonProps to be able to pass properties to the button & menu button, but there is no menuProps to be able to pass properties to the TieredMenu. I wanted to set the popupAlignment value for this component.

Is there interest in this feature ? I can provide a merge request for the code & documentation, it's a simple addition.

We could also make it so that it's possible to extend all three sub components with the pt options, which is not currently possible.

[RolT]

I opened this issue too fast, TieredMenu does not handle popupAlignment anyway (which it probably should). It's still a good idea to provide a way for the user to customize the menu component, but I'll close this issue.