chat: set AttachmentImageItem in python instead of kv: "source" is a StringProperty, and setting it in kv was in some case resulting in a "None" set, which was leading to a crash when Kivy cache was purged. It is now set in Python to avoid this problem.

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .