luajit was failing to GC the notification about 5% of the time. This
commit stores all widget notifications in a weak table and don't let
any lambda access the parent object notification object.
Each of those changes reduces the failure rate. There might still be
a couple in there, but the test passed 200x on my laptop with 100%
success rate.
There is some boilerplate code that make using the widget_template
harder when using the raw `wibox.container.background`. This widget
takes care of it.
Emmanuel Lepage Vallee