Unified Layers

At beginning we mistakenly thought that was better to separate objects per kind because we thought that this way would be simpler for the users and we wanted to promote the use of layers because we think that is is a powerfull feature.


But now we think that is better, and easiest, allow the user decide how he/she wants to distribute the objects across layers.
 
In this way we unified SpriteLayer, TextLayer and 3DModelLayer (not available) into ObjectLayer. Thus, the only different layer at this point is TileLayer that is in fact wholly different from other layers.