[Forwardport] fix: support multiple minisearch widget instances

[DanielRuf]

Original Pull Request

#15485

Description

When cloning the minisearch widget and changing the ID's and selectors they still trigger all instances as the label searchLabel is globally searched instead of in the form.

Manual testing scenarios

1. clone module-search/view/frontend/templates/form.mini.phtml
2. add custom block with the cloned and changed minisearch widget (after changing ID's and selectors in it)
3. click on search icon on mobile

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.