NEW SiteTree->updateCmsListViewForm() to modify the displayed list

I'm aware this adds a bit more clutter to an already cluttered SiteTree, but in the end its consistent with `getCMSFields()`. We could change it into more of a listener pattern, but this implies either new class structures or anonymous functions defined on every invocation.
It would also remove the abilities for inheritance, e.g. to decide on a specific implementation if the parent functionality should be invoked.

The naming is following the new coding conventions with proper camel casing. I think its the lesser of two evils (vs. keeping it consistent with uppercased CMS elsewhere, and creating more deprecations later on).

Docs for the feature are in https://github.com/silverstripe/sapphire/pull/1291