support adding styles to geonode layers

[gubuntu]

• drag / apply a new style to a Geonode layer
  • choose whether to replace an / the existing default / named style or add the new one as a named style
  • if adding the new one a a new named style, choose whether to make it the default style

depends on https://github.com/kartoza/geonode/issues/149

[ismailsunni]

Per @timlinux's request, I made a diagram to show the user interaction when do push and pull style from / to GeoNode.

[ismailsunni]

It seems my diagram won't work. Since we added the layer as WFS, WMS, or XYZ, they are independent from the geonode instance. I am not sure that we can make a variable to register all geonode layer that related to geonode. One example, if I saved a project with WFS/WMS/XYZ layer from geonode, then open it to the other QGIS that doesn't have geonode, it will still work.

And correct me if I am wrong:

1. For WMS layer, since it is a raster, we can't style it like a vector layer. But since the layer in geonode can be vector, it won't be compatible
2. Same goes for XYZ.
3. For WFS layer, the style from the geonode/QGIS Server/geoserver is not applied when we loaded it in QGIS.

One way to do it is to download the layer as it is (vector or raster), the do style manipulation in QGIS. Then we added context menu to push/pull the style or upload the layer. It will be available for all layers in QGIS.

[ismailsunni]

Here I propose another interaction to push layer/style/metadata by using layer context menu. It should work for all layers.

cc @timlinux @myarjunar