Refactor TimeLayerSiderPanel and TimeSlider to function component

[mholthausen]

Description

This refactors the TimeLayerSiderPanel and TimeSlider to a function component.
The property map of TimeLayerSiderPanel will be removed, as it was neither needed nor used.

@terrestris/devs please review

Related issues or pull requests

Pull request type

• Bugfix
• Feature
• Dependency updates
• Code style update (formatting, renaming)
• Refactoring
• Build related changes
• Documentation content changes
• Other (please describe)

Do you introduce a breaking change?

• Yes
• No

Checklist

• I understand and agree that the changes in this PR will be licensed under the
  BSD-2-Clause.
• I have followed the guidelines for contributing.
• The proposed change fits to the content of the Code of Conduct.
• I have added or updated tests and documentation, and the test suite passes (run npm test locally).
• I have added a screenshot/screencast to illustrate the visual output of my update.