Tidy up locking usage around FFTW.

* Move QMutex into the class instead of static.
* Use QMutexLocker
* Only lock around FFTW plan & destroy functions.
Sort Grooveshark favorite songs according to "TSFavorited", as such songs have no "Sort" field.
BTW the compare was buggy (didn't compare against song2). This was OK though, as
we stable sort songs, and they are always ordered as far as I know (I remember
adding this sort just in case they will not be sent ordered on day).
Merge pull request #4648 from sobkas/master

PodcastDownloader changes
Automatic merge of translations from Transifex (https://www.transifex.com/projects/p/clementine/resource/clementineplayer)
A better error url
Merge pull request #4651 from sobkas/fix

Fix the parsing of some strange date formats in podcasts, and add a way to limit the number of podcast episodes shown, and hide listened podcasts.  Fixes #3696, fixes #3475
Produce a proper url for date parse error