Cannot add class to MultiSelect

[TheophileMot]

I would like to add a custom class to a MultiSelect component, thus:

<MultiSelect
  class="asdf"
  ...
/>

When I do so, however, the class does not show up in the DOM.

[metonym]

More generally, this demonstrates the following inconsistent behavior in this library:

• for some components, $$restProps is forwarded to the top most element
• other components pass $$restProps to the "most significant" element (i.e., a nested input)
  • the workaround is to then define ad hoc props like "modalClass" to pass to specific elements

[metonym]

I'm just making a note here about the component API design for v1.

I believe Carbon v11 now always forwards "rest props" to the top-most elements for consistency. We should also consider a consistent practice, and defining additional objects for sub-element props (e.g., inner modal, inputs).

CC @theetrain