Allow mods to insert or delete individual resource icons more easily.

In r18964 / rP18964 code was made resource agnostic except a few places.
This fixes one of them.
(Notice that this approach also addresses Vladislavs recommendation to load all textures at XML parsing time rather than at JS runtime (D1745).)

Ticket: #3934
Patch by: @elexis
Differential revision: D2329
Reviewed by: @Freagarach, @wraitii