SelectMulti improve support for non-scalar lists

[elbakerino]

Currently the @ui-schema/ds-material package widget SelectMulti only supports selecting scalar values correctly.

But also e.g. objects would be valid for enum.const/enum.

• if items.type is set, this should influence the widget logic, if not given it should asume scalar values
• if scalar, it should behave like currently and sort the selected list on change
• if non-scalar
  • there must be additional matching added for "getting from selected value back to oneOf/enum values"
  • pure equality checks wouldn't be good for DX/interop, as e.g. properties would need to be on same place for some and not for other devs
  • typings/docs need adjustments