quick_frontend (app, widget): CagouWidget.onSelected is called when the widget is selected: A property is now used to manage `selected_widget` in QuickApp, and widget's onSelected method is called when suitable.

# XXX: the Monkey Patch is here and not in src/__init__ to avoir issues with pyjamas compilation
# TODO: remove this when changes are merged in Wokkel
from sat_tmp.wokkel import install
install()