xmlui: many improvments: - use a white background to be coherent with the rest of the UI - cancel button is added on "window" XMLUIs - use color for submit/save buttons - VerticalContainer is not a ScrollView anymore, XMLUIPanel now has the main ScrollView - when a TabsContainer is used as main container, main XMLUIPanel ScrollView touch events are disabled, to improve user experience (Kivy doesn't support well ScrollView inside an other ScrollView) - use BoxLayout instead of GridLayout in AdvancedList*, as they use one row/col only - fixed DividerWidget line location - added margins in XMLUI panels - renamed Settings main class to CagouSettings, to avoid conflict with Kivy's Settings class (which is causing troubles with kv)