Add back support for custom picture urls in cards.xml

If a card declares a specific image for a set in cards.xml, use that as the card picture url.
If no custom picture url is defined, use the normal way to guess one.
Both "picURL" and "picURLHq" are supported; the used one depends on the current value of the "Download high-quality card pictures" option introduced in summer magic.
Should fix #154 and #188 (unless the loading of tokens.xml has more problems that needs solving)