Fixed types in FilterableMultiSelect

[guidari]

Closes #16491

Fixed types with wrong validation.

Changelog

New

• Added the types in one file to share between MultiSelect and FilterableMultiSelect

Changed

• Change name from Item to ItemType to match the type in MultiSelect

Testing / Reviewing

• Make sure the MultiSelect and FilterableMultiSelect are working as expected
• Make sure types make sense.

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