backport: added a new "backport" module for using unreleased code from Kivy: Carousel has been backported from Kivy 2.0, because a couple of bugs hitting Cagou are fixed there (notably https://github.com/kivy/kivy/issues/6370). The issue was specially visible when sliding chat widgets. If a version >= 2.0 of kivy is used, a warning will be displayed to indicated that the backport can be removed.