Make extension compatible to TYPO3 6.2

This pull request adds compatibility for TYPO3 6.2 and fixes the subdir wizzard and the preview tab in the plugin. Also the description field has been re-enabled, since I do not get any fatal error in TYPO3 6.2 with that field.

I don't know why GitHub thinks I changed the file class.tx_chgallery_tca.php completely. Actually I just added a single line with the FAL check and the require_once.