core (Cagou): add debug log for widget visibility + ignore_missing: - debug log are now use to show which widget are made (in)visible, and to show the list of currently visible widgets - "ignore_missing" can be used in _removeVisibleWidget when we can expect that a widget is missing in visible widgets list.

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