v0.8.0+beta02

• Upgrade: Compose to beta02. (#39, #47)
• Completely rewrote most of the internals and APIs: (#50)
  • All the actual transition logic is moved out of the Backstack composable and into TransitionController.
  • BackstackInspector and InspectionGestureDetector are replaced with a single API in a new, dedicated module: xrayed().
  • Backstack now just handles the container and managing saveable state.
  • Introduces FrameController, an abstraction which can be used to implement both transitions, the inspector functionality, and things like #17.
  • Removes the spring animations from the inspector. They added unnecessary lag to the gestures.
  • Adds more tests for state handling.
  • Fixed some races that caused navigation animation glitches.
• Use Material DropdownMenu for spinner. (#51)