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.

--- a/cagou/core/share_widget.py	Fri Jan 17 20:35:57 2020 +0100
+++ b/cagou/core/share_widget.py	Sat Jan 18 23:12:52 2020 +0100
@@ -157,5 +157,4 @@
 
     def key_input(self, window, key, scancode, codepoint, modifier):
         if key == 27:
-            self.close()
-            return True
+            return G.local_platform.on_key_back_share(self)