sync #7228 F Show error if form element with same name and class already exists

See merge request !116

F7223allow in filenames

See merge request !114

Marc: hat Carsten getippt

See merge request !112

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.

FormAction.php, QuickFormQuery.php: fixed new implemented error - calling 'Evaluate->parse()' with ROW_EXPECT_0_1 returns only an array, not an array of arrays.

stat(): throws an exception, if file does not exsist - implement file_exist() before doing a stat().

ErrorHandler.php: raise an error as an exception has been stopped in mid 2017 - don't know why, reactivated now.

Fix problem in upload elements: a) in exception FE_ID was not reported, b) fillStoreVar was not fired.

Evaluate.php, QuickFormQuery.php: 1) split exception in user/support, 2) start to implement that '!' in '{{!...' is not necessary anymore. Implemented for 'fillStoreVar' in FE Action elements.