group messages aren't backed up during Export (likely because they're all MMS)

Created by: bobodod

To reproduce:

1. Create a group and send them a couple messages.
2. Navigate to Menu → Import / export → Export tab → Export plaintext backup
3. Evaluate the end of the XML file exported to the root of the SD card and gape with minimal horror at the missing messages.

It's probable that this is because MMS, even plain text ones, aren't backed up and group messages are all MMS.