Load from platform in separate thread

This branch is based on `adapt-load-from-platform-to-new-db`, and this PR substitutes https://github.com/gem/qt-experiments/pull/58, adding to it the feature to run the "load from platform" functionality in a separate thread, interacting with the main thread through signals.
I see some duplication between  get_themes and get_subthemes_by_theme. Probably they can unified.