Use fpcalc to avoid FFmpeg crashes bringing the application down

[Lukáš Lalinský]

FFmpeg occasionally crashes while decoding a file. We should switch to using fpcalc in a sub-process, so that when it happens, we will only know that we can't decode the file, but the GUI application will continue running.

https://groups.google.com/d/msg/acoustid/aJRH4NYm9sQ/E30-oVpvXJkJ