Decompiled published credits window.

Renamed window_credits to window_music_credits.
Added draw_string_centred_underline to gfx.c.
Merge pull request #489 from hexdec/window_publisher_credits

Decompiled publisher credits window.
Still searching for bug
Started reverse of load_object
Added more of object load. Remember to change the openedEntry code
Still trying to find bug