Improve room dispose handling of items.

Currently when a room is told to dispose of old items, it uses the plPassive list to determine what needs to be deleted, however this list also contains objects like trees, torches etc. This means rooms with large numbers of passive objects tend to fill up with a lot of items as the room tries to dispose of the same passive objects over and over.

This pull request changes the DisposeTimer function in room.kod to first separate items out from the plPassive list, then use that list to determine which items need to be deleted. I've also lowered the default delay to 2.5 minutes from 3. Rooms will also only attempt to delete old items if the plPassive list is bigger than 15, to reduce this running in rooms with few objects.

This will increase the amount of list nodes generated during item disposal but should also improve the rate at which items are removed from the floor. Boss rooms use their own DisposeTimer code which still needs to be fixed.