Add `sqlLog` and `sqlLogMode` to QFQ tt-content records.
Add mode 'error' and `none` to sqlLogMode.
Manual.rst: Added explanations for SQL_LOG, SQL_LOG_MODE, and tt-content pendants sqlLog, sqlLogMode. Update config.qfq.ini to latest attributes.
Database.php: rename $mode to $currentQueryMode to make it more descriptive. Recode dbLog().
Logger.php: do nothing if there is no file defined.
Report.php: new function checkUpdateLog().
Config.php: Set defaults for config.qfq.ini SQL_LOG and SQL_LOG_MODE
Store.php: Fix problem that an empty SQL_LOG will be prependad with SYSTEM_PATH_EXT.