Migrate existing values when changing the multiselect separator for non-relational list properties

When editing a class and changing the multiselect separator used in a list property that is not relationally stored (i.e. that is stored in a LargeStringProperty, delimited by separator(s)), the existing values are not updated, so anyone editing objects after the change is made will have unexpected results.

Example:
1. Stored: a|b|c -> a list with 3 elements: [a, b, c]
2. Change separator to ","
3. a|b|c -> a list with 1 element: [a|b|c] (because the stored value's separator is still the old one and the ListClass.fromString() method can not properly deserialize the stored data into multiple elements.