Add stepForward() and stepBack() functions to Scripting API

[#46375033]
Make ScriptingAPI functions public properties of ScriptingAPI object

[#46375033]

Previously they were available only in global namespace.
Now it's possible to access these functions without referring to the
global namespace, e.g.:

var scriptingAPI = new ScriptingAPI(...);
scriptingAPI.api.start()

It will be useful for interactive components, which can use this api.
Add basic version of new playback controller (interactive component)

[#46375033]

It's styled to match Upstatement design. Also now it's implemented as
independent interactive component. To control model, it uses
ScriptingAPI functions.