Properties in C++

Once you have done any development in a language that natively supports properties, like Python or C#, going back to C++ and not having them often feels like a real pain. I've just proposed my second attempt at C++ properties for the nux library.

This change leans heavily on a paper written by Lois Goldthwaite: SC22/WG21/N1615 - C++ Properties -- a Library Solution. I added change notifications using sigc++. I found that using sigc::slot was nicer than templating the properties on the class and member function pointer. This also meant that I could provide a way for a simple property to get its own custom setter method while still having a sensible default.

Compiling C++ templates still gives absolutely horrendous error message that can take a while to mentally parse. I guess one advantage of having done a lot of template programming in the past is that I don't get too phased by copious quantities of error messages, especially for templates, as for one example today, I had just forgotten to change a template arg in a test function, and got way too many lines to sensibly look at. One benefit of that was it caused me to look at what I was doing, and I ended up simplifying my tests a little more.

Thank you Lois for the time you spent writing up the C++ properties proposal, it was a fantastic starting point for me.