Flags and adjacency work (#20259)

Added a bunch of flags for passflags on things that were previously
missing them.
Removed snowflake passthrow var, it's a passflag now.
Updated ClickCross and Adjacency code.
It's now possible to interact with things that are adjacent but
otherwise have a machine (or other appropriate items) blocking them, eg.
think of an APC on the wall with a machine in front of it.

code/game/objects/structures/plasticflaps.dm

@@ -3,8 +3,9 @@
 	desc = "Completely impassable - or are they?"
 	icon = 'icons/obj/structure/plasticflaps.dmi'
 	icon_state = "plasticflaps_preview"
-	density = 0
-	anchored = 1
+	density = FALSE
+	anchored = TRUE
+	pass_flags_self = PASSSTRUCTURE | PASSFLAPS
 	layer = UNDERDOOR
 	explosion_resistance = 5
 	build_amt = 4