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.

f95d0c31d45558c7b3e2fc39398f51eb0190bdca 0.7.0a1
4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0