Add labels for self-modified parts of sprite plotters.

It's unclear to me presently how to get references to these to be
generated.
Add outdoors mask labels for the benefit of generated .asm.
Add static graphic labels for the benefit of generated .asm.
Add sprite labels for the benefit of generated .asm.
vischar_initial: Define structure members.
movable_items: Define structure members.
movable_item_reset_data: Define structure members.
Add character related data labels for the benefit of generated .asm.
screen: Define structure members.
Add message queue labels for the benefit of generated .asm.
expand_object: Write self modifying ops as '<instruction address> + 1'.

Add more labels.
roomdef_bounds: These are no longer only suspected room bounds.
solitary_transition_thing -> solitary_pos.
check_for_pick_up_keypress: Add label for exit.
door_related: Add label at end of block as this address is referenced.