Made Manager a python class. It is no longer an EventDispatcher.
Refactored Manager and Dialog.
Moved dialog to manager module, renamed dialog to manager.
Removed old references to "dialog".
Made batch a property and renamed ManagerGroup to ViewerManagerGroup.
Made root_group a read-only property of ViewerManager.
Made is_pressed of TwoStateController a property instead of a method.
Replaced ContinuousStateController's "get_value" by property "value".
Removed set_options from Selector.
Renamed ViewerManagerGroup attributes to more explicit names.
Made ControllerManager controllers a read-only property.
Rephrased some text in api docs.
Removed old docstring.
Improved controller docs and added new controllers documentation.
Added documentation for ViewerManager and ControllerManager.