set: allow retrieving allowed values for individual properties * libinterp/corefcn/graphics.in.h: add "std::string value_as_string (std::string)" method to (base)graphics_object classes * libinterp/corefcn/graphics.cc: implement base_graphics_object::value_as_string method. * libinterp/corefcn/graphics.cc: set: when 2 arguments are provided, if the second is a property name, either return a cell array containing allowed values or print them.

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