Brings #1066 up-to-date.

• move old gamestate
• create demo (openage.main.tests.engine_demo)
• implement pong
  • calculated with the event engine
  • displayed with the new renderer
  • in a stupid non-event-based-loop (that still processes events but ticks regularily)
  • define minimal gamestate for displaying entities
  • configure pong with nyan
  • define basic nyan api for entity display
  • use the new renderer to display the basic entity
  • prepare abilities and actions (e.g. movement) for the basic entity

once that is done (probably separate pull):

• create basic presenter that is decoupling the simulation from display
• update the pong simulation to run in a non-stupid loop (that sleeps dynamically)