Create an extensible feature architecture for the viewer

[FlorianDe]

• A feature is something the viewer can disable/enable and set the corresponding values (later in dev dat.gui).
• A viewer should be able to register/unregister features.
• The viewer-core should also already have a set of main features implemented.
• It should be possible to use the viewer as a dependency and add custom features by third parties too.
• If possible features should be able to depend on one or another.
• The values of a feature can be seen as a state which should be made observable by others.