quick frontend (app): added "explicit_close" to deleteWidget, and use it on mucRoomLeftHandler: This flag can be used by frontends to decide if the widget must be really closed or not (otherwise, it may be interesting to keep a widget in memory to avoir recreating a new one each time that the chat must be displayed).

# 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()