Merge changes from topics "stl-content-picker", "stl-content-state", "stl-static-picker", "stl-transition-state" into main

* changes:
  Allow to specify transitions between contents
  Introduce StaticElementContentPicker (1/2)
  Rename ElementScenePicker to ElementContentPicker (1/2)
  Use ContentState.Transition in Element.kt
  Extract ContentState out of TransitionState (1/2)
  Move TransitionState into its own file (1/2)