States | Assets loading with states management

[voxelias]

Game should be able to preload assets while displaying a splash screen with progress bar

AC:

• [ ] Demo loads minimal assets synchronously on start in the main thread and stores them in Assets service
• [ ] Demo schedules loading of assets on background with displaying progress
• [ ] One-time execution for loaders should be provided by State, not by task itself
• [ ] Once all assets ready a game scene must be displayed