gaLayers stores topic object instead of topic id
gaLayersChange events include a labelsOnly flag
Add a background property to layers
New implementation of gaLayers.getBackgroundLayers

Uses the topic's backgroundLayers property.
Bg layer directive sets background to true on layers
Background layer directive refactoring

The directive no longer knows about topics.
New layer filter implementation
Fix bg layer and map tests
Merge pull request #393 from geoadmin/dev_bg-layer

Changing services for backgroundlayers