Fixes #2712 (#2797)

When checking unknown sets, consider sets with an empty long name, 
set type and release date as 'incomplete'/'partial' sets.
Do not mark partial sets as known, or ask the user to enable them.
Instead, silently enable partial sets.
When a partial set becomes complete, the user is prompted to enable the
set as before. If they choose not to enable, those partial sets are 
disabled, as we can no longer assume they are disabled by default.

cockatrice/src/carddatabase.h

@@ -43,6 +43,8 @@ public:
     void setEnabled(bool _enabled);
     bool getIsKnown() const { return isknown; }
     void setIsKnown(bool _isknown);
+    //Determine incomplete sets.
+    bool getIsKnownIgnored() const { return longName.length() + setType.length() + releaseDate.toString().length() == 0 ; }
 };
 
 class SetList : public QList<CardSet *> {