changed custom artwork list to tree structure similar to level sets

changed custom artwork list to tree structure similar to level sets

Before, all available custom artwork sets were presented as a long
list in the "custom artwork" setup menu. Now, all custom artwork
sets that are part of level sets (including pseudo level sets that
only contain custom artwork) are presented as a tree structure
similar to the level set selection menu, so it is much easier now
to find a custom artwork set related to a level set.