Split complex `accept_eula` method into several parts

- Improves `Registration::UI::AddonEulaDialog` status from `C` to `A`, see https://codeclimate.com/github/yast/yast-registration/compare/AddonEulaDialog_refactoring

- The change looks quite big, but mainly there are indentation (whitespace) changes, use `git diff --ignore-space-change origin/master...origin/AddonEulaDialog_refactoring` to see the real changes.
I do not get what is this action? I think there should be better name like `cancel_action` or something like that