Slick 3.x breaking changes

Problem/Motivation

At initial Slick 2.x release, we had intention to improve this module along the way:
https://www.drupal.org/project/slick/releases/7.x-2.0

Slick reaches an initial full release!
As an initial, it is the beginning for the more solidified things.
Like any evolving-to-perfection product, Slick is not a perfect product at this initial stage.
Yet it is fairly solid to meet quite a few of RWD slideshow/carousel needs.
The cacheability and lazyload are marking this module worth production.

Slick 7.x-3.x was created as a direct backport of 8.x. It is now dependent on Blazy to take advantage of advanced lazyloading, reduce dry stuffs, a few more improvements here and there, and above all to solidify things as mentioned above.

As it is a complete rewrite, it may not be compatible with the previous branch.

Migration/ upgrading from 2.x to 3.x is NOT supported, yet. Read more.

Proposed resolution

[x] Identify breaking changes, and make this a parent issue to fix a few breaking changes to gradually support the upgrade path if possible at all.
https://www.drupal.org/node/3031759

Remaining tasks

• [x] Write a change record for easy reading.
  https://www.drupal.org/node/3031759
  https://git.drupalcode.org/project/slick/blob/7.x-3.x/docs/UPGRADE.md
• [x] Create child issues which affect or block the upgrade, and provide relevant adjustments, or patches.

User interface changes

None, pretty much similar. No longer shiny gradient for form item, though. Now all is light grey.

API changes

https://www.drupal.org/node/3031759
https://git.drupalcode.org/project/slick/blob/7.x-3.x/docs/UPGRADE.md#sl...

The samples are updated to reflect the API changes at Slick extras.

Data model changes

https://www.drupal.org/node/3031759
https://git.drupalcode.org/project/slick/blob/7.x-3.x/docs/UPGRADE.md#sl...

Release notes snippet

https://www.drupal.org/node/3031759
https://git.drupalcode.org/project/slick/blob/7.x-3.x/docs/UPGRADE.md#sl...