Extend support for custom layers

[zod]

Currently it's only possible to add TMS or Overpass queries as custom layers. Some map tiles are only available as vector tiles (MVT) which brouter-web supports, but not for custom layers.

I made a simple mockup how the extended dialog could look like

For some types (Overpass & GeoJSON) the overlay checkbox would always be checked, because they don't make any sense as baselayer. We could also allow to specify additional information based on the type (e.g. maxzoom for raster layers).