Scan the code for `Layer`s and `control`

[BibMartin]

In PR #293 , we introduce a Layer object that enables to define whether a Feature shall appear in layer_controls (with the optional argument control).

This needs to be generalized to all features (Marker, ...)

[Conengmo]

I think this issue can be closed. The current way of layers and features seems good to me. If we want to do something different in this aspect, a new proposal would be best I think.