Add wreath texture and snow changing code to Qormas.

Qormas now changes the texture on doors (to add textures with wreaths)
and also calls StartSnow in room.kod if necessary which handles snow for
that room. EndSnow removes it.
Add sector/sidedef IDs for placing xmas decorations.

ID 15 is for sidedefs that require a snow texture, Sector ID 10 is for
water which doesn't get snow, IDs 21-34 are for door texture changes.