Werk #16769: MultipleChoice formspec: use list as default value instead of tuple

During the creation of certain rules with MultipleChoice formspec, the following warning appeared due to default values being saved as a list instead of a tuple: "Unable to read current options of this rule. Falling back to default values. When saving this rule now, your previous settings will be overwritten. The problem was: [...]: The datatype must be list, but is tuple."

This werk corrects this behavior by allowing the use of any Sequence of strings as values.

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