Support PronounConcept in icu::message2

[grhoten]

There should be a way to support PronounConcept. Since Message Format 2 does not provide a way to structure data with properties, I guess it will have to be a separate variable. Supporting it completely should take the following into consideration.

1. Be able to specify a hard coded pronoun, like "their", and allow it to be inflected to "his" or "her" or something like that depending on the gender of a person meant for the pronoun.
2. If a PronounConcept has a default gender associated with it, be able to inflect the pronoun like the other objects derived from SemanticFeatureConceptBase. So you could inflect "I" to "we" if inflected to be plural. This is less natural for a translator to use because it requires linguistic knowledge. A more natural way to use it would be to use a known static pronoun, and inflect it based on PronounConcept being a vessel for storing such information.
3. Be able to use a referencedConcept with toSpeakableString to handle the pronouns that depend on the grammatical properties of the referencedConcept. So instead of using the gender of the possessor, you use the gender of the possessed. If you look at the pronoun csv files, this data is represented as dependency=.
4. Be able to use a PronounConcept in a selector. You should be able to query the gender of the pronoun as necessary.

PronounConcept is in reference to people. It is not meant to be used for demonstrative pronouns, like "this" or "that". It's not meant for interrogative pronouns, like "who" or "what". It is meant to be used for human in the first person (the speaker/creator of the message), the second person (the message receiver/audience), the third person (other people), and other objects (animals, or inanimate objects).