Load backgrounds from scene files about thetawave

[cdsupina]

Arena border barriers are also constant they can be defined in scene files as well.

[cdsupina]

It seems that at this point scene files aren't meant for/ aren't capable of handling entities at the complexity level needed. If this changes we will switch to scene files, otherwise we can store the data to define level backgrounds in .ron game data files.

Discussion:
https://github.com/thetawavegame/thetawave/blob/cdsupina/load_background_scene/src/main.rs