Fix object leaks in database code

Qt's database and query model requires that both the queries and the database
objects are out of scope before a database connection can safely be removed.

Solution: Properly encapsulate databases and queries in "{ }" and use a string
to cache the connection name for out-of-scope removal.

YACReaderLibrary/db/comic_model.h

@@ -150,8 +150,7 @@ private:
     QList<ComicItem *> _data;
 
     QString _databasePath;
-
-    QSqlDatabase dbTransaction;
+    QString _databaseConnection;
 
     bool enableResorting;
     Mode mode;