Refactor the application, introduce MainScene, ControlPanel, and UrlHandler objects

[crocodele]

This PR is hard to review in a meaningful way, but here's a summary of the changes:

• A new MainScene component is now the place where the stars, nebula, planet etc. are created, instead of everything being crammed into Planet.
• App is the top-most component, the container for everything else.
• ControlPanel provides the functionality for the right-hand control panel.
• UrlHandler contains the logic for reading from and writing to the URL and browser history.