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.
Perhaps the numbers 30, 30 could be passed parameters (with defaults) rather than hard-coded?