Implement sending segmented chat messages

I am writing PyQt GUI for Hangups and I have needed to be able to send chat messages with segments (to be able to send URLs, format parts of message, etc.). So I have implemented sendchatmessage_segments() in client.py and changed sendchatmessage() to use this method.

There are also other needed changes in pblite.py and schemas.py (serialization of SimpleNamespace objects back to Python lists) and in conversation.py and conversation_event.py (support for sending segmented messages).

I am sorry that I have it all in one big commit, but I have developed it without using Git this time (didn't have too much time). I will try to make it better next time ;-)