This is not a general solution because spawns could be a valid tile
type constraint as well as meaning available spawn tiles.
The separation and a clean implementation is still a TODO.

This allows to not use the internal game state directly from client
code.

Also make more state methods private.

* 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