[WIP] Core gamestate implementation

[TheJJ]

Yea...

• 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)

[duanqn]

What is the current status of this work? I saw the blog referring to it as "New Game State 2020" but we are in 2021 now. :)

[heinezen]

@duanqn SFTtech/nyan#84 was/is a blocker currently. When it's merged, we are going to patch this PR up and merge it :)

[heinezen]

Replaced by #1378