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/library_creator.h

@@ -38,7 +38,7 @@ private:
     QString _target;
     QString _sourceFolder; //used for partial updates
     QStringList _nameFilter;
-    QSqlDatabase _database;
+    QString _databaseConnection;
     QList<Folder> _currentPathFolders; //lista de folders en el orden en el que están siendo explorados, el último es el folder actual
     //recursive method
     void create(QDir currentDirectory);