Make properties (color, symbold) of favorites editable

As requested here https://github.com/nextcloud/maps/issues/122#issuecomment-532167553
A new ticket about "additional" fields.

It would be nice to have the possibility to edit

• the color of favorites imports (per category)
• and the icon (currently it is a star)

Maybe the same icon font could be used like here:
https://umap.openstreetmap.fr/de/map/new/#13/50.2181/1.6704
Would help for importing from uMap and take over values.

uMap also seems to export some fields as key value list (into a GPX file):

<?xml version="1.0" encoding="UTF-8"?>
<gpx xmlns="http://www.topografix.com/GPX/1/1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" version="1.1" creator="togpx">
   <metadata />
   <wpt lat="50.200528" lon="1.662197">
      <name>SomeWater</name>
      <desc>name=SomeWater
description=Here is a description about the POI it is some water</desc>
   </wpt>
</gpx>

So looks like the tag contains a key-value list.
An export as geojson is more defined:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "_umap_options": {
          "color": "OliveDrab",
          "iconClass": "Drop",
          "iconUrl": "/uploads/pictogram/hospital-24-white.png",
          "showLabel": true,
          "labelInteractive": true
        },
        "name": "SomeWater",
        "description": "Here is a description about the POI it is some water"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          1.662197,
          50.200528
        ]
      }
    }
  ]
}

No files or tests are named. Start by locating the favorites editing UI and the import handling for GPX and GeoJSON, then trace how category, color, and icon values are stored; done means favorites can edit and retain those properties and imports can use the relevant uMap values.