WIP - Adapt load from platform to new db

The dialog to select socioeconomic indicators has been rebuilt and made much more flexible, improving filters and making the GUI more usable, also thanks to a widget to build a list of selected items.
There is a companion PR on the platform (FIXME missing link), with the new socioeconomic DB and the API to retrieve data from it.

Open issues:
* When no geometries are downloaded, the csv-based layer is not editable (and even attempting to save as shapefile fails)
* We need to explore what happens to the workflow if the data is joined with user-provided geometries