Added automatic generation of inspector layers menu

Layers menu in Frontend can be enabled with ?debug_layers=true similar
to ?debug=true.
Layer names are taken from new API /otp/routers/default/inspector/layers
Changes are minimal. Only descriptive name is added to all inspector
layers and new API resource.
TODO:
There is currently weird output for XML:
```xml
<InspectorLayersList>
  <layers>
    <layers>
      <key>transit-links</key>
      <name>Transit/Bikeshare links</name>
    </layers>
    <layers>
      <key>bike-safety</key>
      <name>Bike safety</name>
    </layers>
  <layers>
</InspectorLayersList>
```
second "layers" should be "layer"