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:
<layers><layers>xy
second "layers" should be "layer"
Merge branch 'master' into inspectorLabels
Added support for automatic legend generation for Wheelchair renderer

Which was added in #1569
Merge pull request #1576 from buma/inspectorLabels

Added automatic generation of inspector layers menu