Badges on Tabs

[KDederichs]

Short description of what this feature will allow to do:
It would be neat to have the ability to add badges to form tabs.

Example of how to use this feature

It'd be nice for displaying counts of certain collections in objects.
It'd probably have to be with some sort of callback that has access to the entity instance:

            FormField::addTab('Test Tab')->setBadge(function (Foo $foo) {
               return $foo->bars->count(); 
            }),

something along that line.