Restore custom MultiUnicode field processing
Restore custom MultiUnicode field processing

I removed it earlier in commit 20fd35b, which turned out to be wrong:
custom fields *can* be of type MultiUnicode.

sillsdev/LfMerge,sillsdev/LfMerge,ermshiperete/LfMerge,sillsdev/LfMerge,ermshiperete/LfMerge,ermshiperete/LfMerge