Tweaking the API and cleaning up

[almarklein]

• Use str id's for most components. Closes Reduce noise in the callback graph #9.
• Add example to use a different input component as the slider.
• Clean up the docs.
• Show more config options in one example. Closes Add config parameter to DashVolumeSlicer class #6.
• Add tests.
• Public attributes are now @properties, for clarity and better documenting.