The new `maybeFormatMessage` function detects whether its argument
looks like a message descriptor object and, if so, will call
`formatMessage` on it. This is now used for all user-visible text fields
in extensions.
Also, messages may use "select" to check the target type with a message
like this: '{targetType, select, stage {text for stage} sprite {text for
sprite} other {text for other}'. Note that the "other" clause is
required by `formatMessage`.