New OSM layers should be put on top of layer stack.

[bilbo]

When I create a new layer by downloading some OSM data, it is put at bottom of the stack, but above any WMS layers. However, this puts it usally below the "Validation error" layer, resulting in validation errors obscuring the view (until I manually reporder the layers)

I think new OSM data layers should be put on top of the layer stack by default.

[Gubaer]

(In [2192]) fixed #3529: New OSM layers should be put on top of layer stack.