Leaking frontend widgets

[AntoinePrv]

In xtarget, when a widget is created by the frontend:

xwidgets/src/xtarget.cpp

Line 30 in 1cdc9f3

void xobject_comm_opened(xeus::xcomm&& comm, const xeus::xmessage& msg)

it is added to the registry:

xwidgets/src/xtarget.cpp

Line 55 in 1cdc9f3

factory.make(std::move(comm), state, buffers);

xwidgets/src/xfactory.cpp

Line 34 in 1cdc9f3

m_makers.at(model_module + model_name + view_module + view_name)(std::move(comm), state, buffers);

xwidgets/include/xwidgets/xmaker.hpp

Line 32 in 1cdc9f3

get_transport_registry().register_owning(reinterpret_cast<xmaterialize<CRTP, P...>&&>(model));

But never removed. It should be destroyed when the comm is closed by the frontend.

However the way to do this is not clear. Tracking how to do it in jupyter-xeus/xeus#356