feat: add mobile support for the footer in HFG

[preda-bogdan]

Summary

Allow mobile layout for the Footer Builder in HFG.

Will affect the visual aspect of the product

YES

Screenshots

Footer Desktop/Mobile builder (Pic. 1)

Test instructions

1. On a fresh instance of Neve
2. Go to "Customizer" > "Footer"
3. Check that the "Mobile" tab is available for the builder (see Pic. 1)
4. Check that on the front-end the items from the Desktop are also displayed in Mobile view without changing anything.
5. Change just one of the views (Mobile or Desktop)
6. Check on the front-end that each footer is displayed correctly for the view (Mobile/Desktop)
7. Check that it works as expected with Neve Pro

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes Codeinwp/neve-pro-addon#2504.

[pirate-bot]

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[GrigoreMihai]

I don't see any issues with the code, however there is a test failing that is related to the HFG, I think that is because now we also have the mobile version present same as on this test https://github.com/Codeinwp/neve/pull/4185/files#diff-e6a31b86eae9e18bd070d922c82b1b1ba8ba57166790f571fbd6692d3cb95f10R7?

I will approve it as I think only the test needs to be edited a bit.

[irinelenache]

@preda-bogdan Tested and everything's fine regarding the PR 🚀

[pirate-bot]

🎉 This PR is included in version 3.8.0 🎉

The release is available on GitHub release

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