Vinnyt123/feature/15 mindmap node

[rmoradc]

Implememts #78 Ticket has changed so this pr has a self made mind map functionality as mmp was not viably extendable.

• [ ] Change in CHANGELOG.md described (if applicable)
• [ ] Tests created for changes (if applicable)
• [ ] Manually tested changed features in running JabRef (always required)
• [ ] Screenshots added in PR description (for UI changes)
• [ ] Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.