HelperFormElement.php, SessionCookie.php, Html2Pdf.php, Store.php, Constants.php: migrate to camel case on configuration variables.
Config.php: readConfig() reads now config.qfq.ini and T3 LocalConfiguration.php.

Manual.rst, AbstractException.php, SessionCookie.php, Downoad.php, Error.php, Link.php, Report.php, Store.php, AbstractBuildForm.php, BuildFormBootstrap.php, config.qfq.example.ini: replace all access to SHOW_DEBUG_INFO with respect to multiple values.

Implement additional 'SHOW_DEBUG_INFO = download' to track down problems with 'session forwarding'.
Configure constants for SHOW_DEBUG_INFO for yes|no|auto

Html2Pdf.php: Fake the current 'user-agent' and add it as '--custom-header ...' AND '--custom-header-propagation' option to the wkhtmltopdf call. Without, T3 will deny FE Group access. The --custom-header-propagation seems to be necessary.