core: better back key (ESC) management: - back key (which is mapped to esc keycode by SDL2 backend) is now handler with a platform specific method when on root widget (i.e. a default widget is selected, or nothing is selected). Default behaviour is to do nothing, while on Android the app is put to background - CagouWidget now has a default key_input method which go back to default widget.

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