LayerIndex ehancements

[zbycz]

• When adding a layer which has CORS protection switched on, it would be useful to warn the user, that this can't be seen. (Probably also for custom layer.) Example: "Public Transport (öpnv)" map

• When there are many tileservers offered, we could place some token in URL (like {servers:a,b,c}) and then parse it here. No need to select one tileserver.