Bug fixes, init signals for mobs, density updates. (#21108)

1. Look up now follows your mob (no more auto-removal when you move,
only when you go below a solid tile). Maybe add a hotkey?
2. Made the codebase work with the latest version of spacemandmm lint.
3. Advanced kits don't work on robotic limbs.
4. Density shifting should now always use set_density. Ported
TRAIT_UNDENSE from CM to better control the density of living mobs.
Fixes weird shifting behaviour with wall leaning -- what was happening
is that mob density was getting reset to TRUE because the mob wasn't
lying down.
5. TGUI say light prefs actually save.

---------

Co-authored-by: Matt Atlas <liermattia@gmail.com>

code/modules/integrated_electronics/core/special_pins/list_pin.dm

@@ -23,7 +23,7 @@
 	t += "<hr>"
 	// Iterating by index simplifies editing/deletion in game,
 	// since the href_list["pos"] var is consistent
-	for(var/i = 1, i <= my_list.len; i++)
+	for(var/i = 1; i <= length(my_list); i++)
 		t += "#[i] | [display_data(my_list[i])]  |  "
 		t += "<a href='byond://?src=[REF(src)];edit=1;pos=[i]'>Edit</a>  |  "
 		t += "<a href='byond://?src=[REF(src)];remove=1;pos=[i]'>Remove</a><br>"