chat: resize attachments (images only for now): if attachments to send contain oversized image, a checkbox will be shown (activated by default) to reduce automatically the size. The background color now cover the whole attachments to send widget. If not already specified, media type is guessed from filename when adding an attachment.

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