Clarify the "attached layer" notion and improve layer adding API

[jailln]

There is a notion of "attached layer" in iTowns and by default all layers added to a GlobeView or a PlanarView with the view.addLayer() method are attached to the tileLayer of the Globe or the Plane.

Moreover, the API for adding layers is not very clear in my opinion since to add layers that are not attached we need to use the View addLayer() method with itowns.View.prototype.addLayer.call(view, layer) : which seems to always be the case for C3DTilesLayer and PointCloudLayer by the way. I think we should always use view.addLayer(layer) and think of some parameter or conditions to attach layers that:

• is hidden to users that use itowns and don't want to go into such details
• allows enough flexibility to attach or not layers

Two questions: (ping @gchoqueux @mgermerie )

• What exactly is an "attached layer" and what are the benefits of attaching a layer or not?
• What's your opinion on the API modification?