widget has modify_* (base, fg, bg, text) but doesn't have cursor_color.

it doesn't make much sense to me, because atm if you do a modify_bg to black
then you see that you need the typing cursor to be sth else than black (eg. white)

then you realize that modify_cursor_color does not exist, so you're bummed and
you have to low-level with rcstyles..

in the same spirit for providing all those higher level functions, GTK+ should
also provide one for cursor_color so it is in fact a usable and complete set of
high-level functions that relate to changing colors in a widget