Improve loading screen logic

[leanmendoza]

The current logic can be tricky and introduce the next issue: #253
To change this behaviour the loading screen logic has to be rewritten by:

• Generating well-defined scenes to be loading: today, this is dynamic because pointers are fetched by demand, so a new state defining if all the pointers and urns are loaded has to be created
• Defining a monotonic increasing progress for scenes: now the scenes have several initial ticks to send the GLTF to be loaded, this increases the amount of the total of assets to be loaded to consider if the scene is completed loaded. An assumption of considering the progress random can work, then the challenge is to not let it seem to be stuck in the last step