Refactor LayerTree to a function component

[dnlkoch]

Description

This refactors the LayerTree component to a function component.

BREAKING CHANGES:

• Removed prop map
  • wrap component in MapContext.Provider instead (see example)
• Removed prop toggleOnClick
  • to enable set a default CSS override for .react-geo-layertree > .ant-tree-node-content-wrapper (pointer-events)
• Removed LayerTreeNode
  • most likely no migration necessary as no part of the public API

Related issues or pull requests

Pull request type

• Bugfix
• Feature
• Dependency updates
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• 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.

[simonseyock]

small suggestion, don't know if it's worth it

[dnlkoch]

Thanks for the reviews!