Show member list for private groups

Currently difficult to test since Briar core's TestDataCreator apparently doesn't create private groups and our deterministic test data creator for now only creates empty private groups until #113 (closed) is solved. Can now be tested either with random test data or by starting TestWith{Two,Three}ConnectedTemporaryAccounts.

Draft since based on !313 (merged) !328 (merged)

Closes #495 (closed)