(svn r1503) Added feature:
o allows users to setup the production values of the rawmaterial producing
industries in the editor

Modified:
o ttd.h - added CT_INVALID for 0xFF cargo type
o english.txt - added 1 string
o industry_gui.c - the feature itself
o window.h - added compile asserts to the structs which checks whether their
sizes are smaller than WINDOW_CUSTOM_SIZE

Thanks:
o Darkvater to bother me to constantly improve the patch
o Various users at #openttd for testing