reimplement the invert option as a KSaneOption

This implements the option to invert the color of the images as a KSaneOption derived option, which brings the benefit that it nicely integrates with the rest of the options and no special handling is required anymore. This will make it possible to easily use this option from other applications. It is implemented as derived class which overrides most of the KSaneOption members. KSaneOption is slightly adapted for these changes.