Merge location_t into player_t struct, etc.

Removed the location_t struct and merged its data members into the
player_t struct. Also deleted the "deinit_location" function, added a
"get_inventory_row_for_pebble" function, and made some
corrections/improvements in "show_narration", "show_window",
"menu_select_callback", "init_player", "deinit_player", and
"add_current_selection_to_inventory".