Configure app with JSON based config

[chrismayer]

This reworks the application creation in a way that at the startup an application configuration is loaded. The app config is a JSON document, which holds information to influence the initial state of the application, e.g.:

• Map Center
• Map Zoom
• Layers and their Properties
• Application Title
• Application Logo URL and Size

This has the advantage that the initial state of the application can be changed without building and deploying the app.

The JSON looks like:

{

  "title": "Vue.js / OpenLayers WebGIS",

  "logo": "https://www.placecage.com/100/100",
  "logoSize": "200",

  "mapZoom": 2,
  "mapCenter": [0, 0],

  "mapLayers": [
    {
      "type": "WMS",
      "lid": "ahocevar-wms",
      "name": "WMS (ahocevar)",
      "format": "image/png",
      "layers": "topp:states",
      "url": "https://ahocevar.com/geoserver/wms",
      "transparent": true,
      "singleTile": false,
      "projection": "EPSG:3857",
      "attribution": "",
      "isBaseLayer": false,
      "visibility": false,
      "displayInLayerList": true
    }
    //...
  ]
}