P3-321 improve Metabox class

[enricobattocchi]

Context

• The current implementation of the Metabox class has been copied directly from the legacy code. It uses WP function and hooks in a wrong way and forces to use JS to hide the custom metabox for post that are not copies.

Summary

This PR can be summarized in the following changelog entry:

• Refactors the Metabox class.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• visit Settings > Duplicate Post, third tab Display, and make sure the Show original item: > In a metabox in the Edit screen option is enabled.
• Make sure you have at least:
  • 1 post which is not a copy of anything
  • 1 post which is a R&R copy of another post
  • 2 posts which are regular copy of other posts
  • 1 page which is a regular copy of another page
• perform the following steps both in Block and Classic editor
• Edit a post which is not a copy of anything, and see that you don't see any Duplicate Post metabox. Please make sure the metabox is not disabled (Block editor: Options > Preferences > Panels, Classic editor: Screen options). Note by Andrea: just to clarify these test instructions for future reference: in this case, there's no setting related to the Duplicate Post meta box under "Block editor: Options > Preferences > Panels, Classic editor: Screen options" and that's the expected behavior. The setting is only displayed for posts that do have a copy.
• Edit a post which is a R&R copy of a post, and see that you see a Duplicate Post metabox like this one (Classic editor view, in Block Editor the content will be the same)
  
• Edit a post which is a regular copy of a post, and see that you see a Duplicate Post metabox like this one (Classic editor view, in Block Editor the content will be the same):
  
• tick the checkbox and save the post
  • if you are on Block Editor, reload the page (F5) - on Classic Editor this happens automatically
  • see that the Metabox is not there anymore
• visit Settings > Duplicate Post, second tab Permissions, and disable the plugin for the Page type
• edit a page which is a copy of another page and see that you can't see the metabox at all
• visit Settings > Duplicate Post, third tab Display, and make sure the Show original item: > In a metabox in the Edit screen option is disabled.
• edit a post which is a copy of another page and see that you can't see the metabox at all

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes [P3-321]

[enricobattocchi]

Note to whoever will be accepting this: do not merge until the creation of release/4.1.2

[jcomack]

CR done 👍

[afercia]

Acceptance 👍