core (widgets handler): refactoring: - CagouWidget now has class properties (to be overridden when needed) which indicate how if the widget handle must add a wrapping ScreenManager (global_screen_manager) or show all instances of the class in a Carousel (collection_carousel). If none of those options is used, a ScrollView will be wrapping the widget, to be sure that the widget will be resized correctly when necessary (without it, the widget could still be drawn in the backround when the size is too small and overflow on the WidgetWrapper, this would be the case with WidgetSelector) - some helper methods/properties have been added to CagouWidget. Check docstrings for details - better handling of (in)visible widget in WidgetsHandler - thanks to the new wrapping ScrollView, WidgetSelect will show scroll bars if the available space is too small. - bugs fixes

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