Added Inventory.AlwaysRespawn flag.

This adds the Inventory.AlwaysRespawn flag, not to be confused with AlwaysRespawn for monsters. In the case that both Inventory.NeverRespawn and Inventory.AlwaysRespawn are specified, the item will not respawn. Forum thread: http://forum.zdoom.org/viewtopic.php?f=34&t=45882