Add obj_get_string and obj_set_string

[anatoliy-savchak]

I need that to set PC name from a script.

Although only obj_f_pc_player_name would be used for, but it would be nice to have for consistency as well.

Please approve.

[DudeMcDude]

I don't object in principle, just make sure it doesn't cause references to strings that might be freed python-side.