widgets_handler: work around the double call of onSlideChange: with Kivy dev version a95d67f (the one currently used in Python for Android), onSlideChange which is binded on current_slide was called twice with the same widget, resulting in widgets unexpectedly deleted. This patch fixes it by abording the callback if new_slide is the same as _former_slide, i.e. if the widget is not actually changed. Started a discussion with upstream Kivy team to check if this is a bug or feature.

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