core (widgets handler): clear widgets when WHWrapper is detached:
when WHWrapper is detached from parent, we call clear_widgets to detach children. This
avoid CagouWidget.whwrapper to lead to this now unused wrapper (this was causing a bug
when testing if a widget is attached and could be re-used, leading to the unnecessary
creation of new widgets).
Added ignore_missing when removing current_slide from visible widgets, to avoid error
logging if wrapper was not active.