Add MAGE graphics, update Pebble names, etc.

Added code for MAGE graphics in "draw_cell_contents", put the "level up"
check back into "damage_npc", adjusted "determine_npc_behavior",
"cast_spell_on_npc", "tick_handler",
"level_up_menu_draw_header_callback", "strcat_item_name", and
"init_npc", changed the names of two Pebble/magic types ("Lightning" to
"Thunder" and "Darkness" to "Shadow"), and adjusted the ordering of
Pebble, stat, and status effect constants.