reorganized layers with proper sorting
reorganized layers with proper sorting

background -> midground -> textground -> [foreground] -> overlay.
 background is for sprites
 midground is for characters and props (almost everything really)
 textground is for callouts
 overlay is for title screens.
foreground is not yet implemented, and I doubt we would need it.
top-level groups (layers) are sorted manually, while sub-groups are
y-axis sorted.

The current player's text callout will hop to the top of the
textground layer when they become active.

james-storey/LD29