[12535] Loot-System: Fix reference loading check

This fixes a false error output for loot references that are only used with spell loot.

Thanks to X-Savior for properly reporting both error messages and use case
[12536] Fix use of config values related to quest-status and level

This fixes use of negative value in config values Quests.LowLevelHideDiff and Quests.HighLevelHideDiff

Also add some documentation around the related code

Thanks to Neotmiren for pointing and to cala for testing.
[12537] Immediately remove corpses when ForcedDespawn is used

Thanks to Neotmiren for pointing, special thanks to cala for testing!

Close cmangos/issues#222
[mr2862] define two transport-related DBC structures