Client connecting to #338 of lizard-nxt

Problem
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Events are treated non-universally when compared to standard Leaflet layers.

Solution
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Treat event layers the same as regular Leaflet layers as much as possible.

Todo
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- [x] Django bootstrapping of the correct value for window.data_layers
- [x] Process window.data_layers correclty in the Angular code
- [x] Tidy up all connections in front-end: layers vs. layerGroups  <---> WMS vs. TMS vs. UTFGrid
~~- [ ] Extract logic for (Leaflet) vector layers to Angular service~~ This will be another PR

Requirements
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lizard-nxt:  https://github.com/nens/lizard-nxt/pull/338
hydra-core: https://github.com/nens/hydra-core/pull/102