Parameter typing and return type hints

Well, that didn't break anything...but there are still some object hints that I want to convert to proper class/interface names, once I figure out what those should be.

So apparently declare(strict_types=1); on the Table plugin causes a lot of problems, almost certainly because of the parent class (core Drupal Views module Table) doesn't use strict_types and has some typing problems.

I think there are already a lot of changes in this MR and I want to get this much in - none of these changes breaks anything, so it will make a good new baseline from which we can try to address the strict_types.

Automatically closed - issue fixed for 2 weeks with no activity.