GeoJSON source creation and editing

[martinRenou]

As a follow-up of #30:

• Look into reusing the FileBrowser widget from JupyterLab for file selection, instead of typing the raw file name in a text input
• When creating a GeoJSON layer from a file, show a "readonly" checkbox that will set if we can edit or not the local GeoJSON file.
  • Provide a codemirror input in the object properties panel to allow editing the file/GeoJSON content
  • Provide UI widgets using https://github.com/mapbox/mapbox-gl-draw for editing and creating GeoJSON features
• GeoJSON source from url
• Bring back the GeoJSON icon Add geoJSON source and layer #30 (comment)