Removes `sealed` modifier from `DataListItem` class

[soreng]

Description

As discussed in #279

Removed sealed modifier from DataListItem.

Related Issues?

279

Types of changes

• [ ] Documentation change
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the coding style of this project.
• [x] My changes generate no new warnings.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the corresponding documentation.
• [x] I have read the CONTRIBUTING and CODE_OF_CONDUCT documents.

[leekelleher]

Thanks @soreng!

Resolves #279.

[umbrabot]

Hi there @soreng!

First of all: A big #H5YR for making an Umbraco related contribution during Hacktoberfest! We are very thankful for the huge amount of PRs submitted, and all the amazing work you've been doing 🥇

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