clean up item slot / tooltip rendering -- only render either the items
tooltip or a custom tooltip.  Also, fix depth-test for drawing of
tooltip background rect.
add hint icons to npc inventory GUI to denote what goes in each
inventory slot
update npc base to add canWander flag.  used in wander AI to check if
npc can wander.  set via creative-control GUI for structure npc setup.
Not saved/persisten, not meant to be used in survival mode
update faction based NPCs to have follow-player AI, for use in creative
mode for structure setup / npc positioning.  
also allow access to inventory / advanced controls for faction-based
npcs for creative mode players -- 'craft-an-npc' step 1