Allow Modifiable to extend a super class

[marquiswang]

With Builders, we can add a static nested class named Builder that will be extended by a generated Builder.

There is no equivalent for Modifiable. Therefore, if we want to use Modifiable to simulate smart builders, there is no way to achieve the same functionality.

[mphalpinpfpt]

Seconding this. I made a similiar request a couple months ago, issue 1102.

[kiddkaffeine]

Opened a PR for this