Sets handling improvements

This PR includes misc improvements to sets handling:
 1. "TK" has been unified as a static CardDatabase member for better readability;
 2. oracle always adds a "TK" set with a proper description; 
 3. oracle now saves to cards.xml two additional fields: set type and set release date;
 4. the "edit sets" dialog shows these new fields as new columns;
 4. the "edit sets" dialog is sortable;
 5. the "edit sets" dialog has 2 new buttons to save the current shown sets order as the default sets order;

Additional changes that could be needed:
 a. update the value of CardDatabase::versionNeeded since the card database has new fields and old client could worry about it;
 b. when the "edit sets" dialog is closed, invalidate every existing card's cached picture to ensure the picture from the preferred sets is shown (even if previously a card picture had already been cached).
Does the downloading code need to know about the set's release date? I'd only have this type contain the minimal amount of information needed to perform the download