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.
This will be shared by the notifications stored in a wibox/wibar and the
ones using a popup. It extends the constraint and margins container
to take care of some boilerplate code. While other widgets have their
own public API, those 2 are private since they are not different enough
to warrent a new public module.