r1597: Adding custom columns to the bug listing display. Fixes bug://report/73

r1597: Adding custom columns to the bug listing display. Fixes bug://report/73

* includes/definitions.php: Removed
* includes/init.php: Do not include definitions.php anymore
* includes/class_sort.php:
(ListSorter::fetch_by_text): Fetch custom fields when getting the text information
(ListSorter::constructColumnHeaders): Use fetch_by_text() instead of $bugsys->columnNames[]
(ListSorter::_processColumns): Use fetch_by_text() instead of $bugsys->columnNames[] and make sure that the columns the user has selected actually exist (namely for permissions sanity)
(ListSorter::_fetchCustomFields): New method that returns all the custom fields in the system, or just the ones the user can see, based on a parameter