feat($rootScope): allow $evalAsync to queue operations to be run after digest

[jankuca]

This makes it possible to have callback functions for the digest loop.

The usefulness of this feature can be found when one need to safely measure or check the contents of elements after the DOM is updated.

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[btford]

Rejected with extreme prejudice.

This kind of high level test doesn't belong to $rootScopeSpec.js

We should ensure that Matias's PR #3649 should contain a test like this.