Gamedata job

not blocking UI thread for loading gamedata resources
please swap engine and assetmanager lines to fix this warning:
```
In file included from /home/fd/src/openage/cpp/game_main.cpp:3:0:
/home/fd/src/openage/cpp/game_main.h: In constructor ‘openage::GameMain::GameMain(openage::Engine*)’:
/home/fd/src/openage/cpp/game_main.h:111:19: warning: ‘openage::GameMain::engine’ will be initialized after [-Wreorder]
  openage::Engine *engine;
                   ^
/home/fd/src/openage/cpp/game_main.h:104:15: warning:   ‘openage::AssetManager openage::GameMain::assetmanager’ [-Wreorder]
  AssetManager assetmanager;
               ^
/home/fd/src/openage/cpp/game_main.cpp:99:1: warning:   when initialized here [-Wreorder]
 GameMain::GameMain(Engine *engine)
```