Make the activity stack explicit and move reusable activities to the
activity package.

* Implement move artifact action
* Fix widget update memory leak
  * update the highlights not during on each frame
  * do not update the unchanged label of a button
* Allow to exit the game by pressing 'q'
* Implement some cards from the magic set
* Improve permanent formatting
* Some tweaks to UnitAI code

* Generalize hover detection and move it into ui.Collection
* Implement transparencu for the prompt banner