connect gaLayersService to layersconfig service

1) Connect gaLayers to layersconfig service
2) Move loadForTopic() in TopicDirective
3) Update topic controller
4) Fix wmts format issue

Open points:

At moment the event listener for topic change is in BackgroundLayerSelectorDirective.js

I think it's not ideal because one needs to use loadForTopic() all the time.
One solution would be to create a BackgroundLayerSelectorService and BackgroundLayerSelectorController in which one would listen to 'gaTopicChange'.

Otherwise the BackgroundLayerSelectorDirective will be "topic" dependent.