Picurl handling

These three commits adds the following changes:

* PictureToLoad::getSetName() doesn't crash anymore if a card has no sets.
* CardInfo::getPreferredSet() returns 0 instead of crashing if a card has no sets.
* CardInfo::getPreferredMuId() return 0 if a card has no sets.
* PictureLoader::processLoadQueue() won't try to load a card's picture from folders containing the set name if * the card has no sets.
* PictureLoader::getPicUrl() won't crash trying to get the set name of a card without sets
* PictureLoader::picDownloadFinished() won't try to save a card's picture to disk if the card has no sets.