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.

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .