Fixes #3992 (Feature): dbNameQfq & dbNameData already implemented. dbNameT3...

Fixes #3992 (Feature): dbNameQfq & dbNameData already implemented. dbNameT3 renamed from T3_DB_NAME. No more configuration in config.qfq.php necessary. Manual updated.
AbstractException.php: fixes access to uninitialized array elements.
Download.php: get dbT3 now from STORE_SYSTEM
Config.php: read 'dbNameT3' from TYPO3_CONF_VARS or from T3 config file.
QuickFormQuery.php: refactor gathering 'dbNames' - now from config.qfq.php. Fixes bug in getForwardMode() for QFQ inline editing.
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