Optimizations and improvements...

Fixed asteroid regeneration/ore collection cap. Added a more dynamic way
to play a single sound effect once per level in ContentHandler (without
a bool flag). Switched the root GameObject position holder to center
instead of UL coords, since it is used much more and requires less
conversions. Added a border buffer to NPC generation, so they won't
spawn on the edges, and a UI notification when ore collection maximums
have been reached.