Window:titlebar is optional, but set_titlebar() takes reference, and there's no unset_titlebar()
TSIA really. The titlebar is an optional
Widget*, and is returned that way by the property getters, but
set_titlebar() takes a reference, which obviously is not nullable. We lack an
unset_titlebar(), as usually doing in situations like this, so there's no (C++) way to remove the
:titlebar widget once it has been set once.