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.

f95d0c31d45558c7b3e2fc39398f51eb0190bdca 0.7.0a1
4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0