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.
start implementation of track list window
implement scrollmouseover and scrollpaint for track list window
implement track list window paint
implement window_track_list_select
implement track_get_info
implement sub_67726A, track loading function
Merge branch 'track-list-window'
Merge branch 'master' of github.com:IntelOrca/OpenRCT2