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.