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.
The extension `.csvt` looks strange. Care to comment on its meaning? Otherwise it looks like a mispelling error for ``.csv``.