Avoid crashes if a card has no <set>

Well, all but the last three commits (9427097, 99c7c8e, 5779f06) are just to take your branch up to date.
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.