Move purple-mm-sms into chatty
Having a libpurple plugin is helpful, but currently we use some hacks to track message sending and delivery states. Also, sms accounts doesn't behave the same as IM/chat accounts. Say for example, an SMS thread can have subsequent messages sent by different accounts (ie, with different SIM), or one message can be an SMS and the one followed can be an MMS. Handling various such cases could be harder when staying within the limits of libpurple. So let the purple-mm-sms code be moved into chatty so that we have better control of SMS and MMS messaging