Further Freesound import tweaks.

Make download of sound files multi-threaded. Each sound file download takes
place in its own thread, and has its own progress bar and cancel button,
which stack up from the bottom of the list of results.

Sound files download into a file with a '.part' suffix, which is then
renamed to the intended name on success.

Add a 'Similar' button, which searches Freesound for sounds similar to the
currently-selected sound in the results list.

Add a freesound-download-dir config variable to decide the location
of sound files downloaded from Freesound.

Move Windows-specific logic to replace '/'s with '\'s from
Mootcher::changeWorkingDir() to Mootcher::ensureWorkingDir(), and remove
the now unused Mootcher::changeWorkingDir().

Use Glib::build_filename to construct paths.