render styles (optionally) for inactive layers

[dieterdreist]

Currently, only one data layer is rendered "visibly", the others are very faint and almost invisible, especially with a lot of data in the active layer which covers it.

It would be great to be able to have inactive data layers "visible" (maybe even above the current layer, i.e. respecting layer order in the layer menu), but with the features not selectable (it could be another setting whether you want the layer to be taken into account for snapping or not).