Changing services for backgroundlayers

Please see discussion at #366.

Eric's proposal is the following:
* [1] includes all possible background layers for each topic.
* Remove the "background" flag in [2].
* gaLayers maintains an array of background layer names (updated on gaTopicChange).
* gaLayers.getBackgroundLayers returns an array of ol.layer.Layer instances marked as "background" layers.
* gaLayers broadcasts gaLayersLabelsChange events when the labels only are changed.
* The bg layers directive listens to both gaLayersChange and gaLayersLabelsChange.
* The bg layers directive doesn't know about topics.
* With #342 we need to be careful with layer ids, as we can have multiple layers with the same idBod in the map.

This will require changes in the services as well as in geoadmin3, which would need to be done in parallel. Something to discuss in our next meeting.

[1] http://mf-chsdi30t.bgdi.admin.ch/rest/services
[2] http://mf-chsdi30t.bgdi.admin.ch/rest/services/ech/MapServer/layersconfig