introspection: add get_f()

This function returns a pointer into introspection_linear for the entry
describing the wanted field.
introspection: add support for #include

Only files in iop/ and common/ will be read, other directories contain
files that confuse the parser and they are not needed anyway.
Use introspection to get iccprofile of images ...

... instead of including iop headers. Also make more stuff private to
colorin/colorout iops.
Use area colorpicker instead of spot for picking film base color.