WIP: Barely working zforms support.

[aero31aero]

This is just a proof of concept. I'm not sure where to add what code and I essentially wrote a lot of unneeded functions to export things from the message for now. I'll clean this up in the next few commits.

The current interface is to press a number key from 1 to 9 to select one of the choices available in a message:

This creates an obvious restriction of maximum 9 choices per message. I wonder if a popup menu to choose would be a better UI?