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.
See the companion PR on the platform (https://github.com/gem/oq-platform/pull/268), with the new socioeconomic DB and the API to retrieve data from it. Production data is in the companion PR https://github.com/gem/oq-platform-data/pull/2)
In the current version, when loading socioeconomic data from the platform, also geometries are downloaded (using GADM countries simplified with Bend Simplify algorithm and a tolerance of 1km).

TODO:
* Execute the downloading as a separate thread and implement a progress bar (or similar)