Add CMakeLists.txt and Screenshot

[MoriokaReimen]

Dear Mr. abodelot

Your sfml-widgets project is nifty and caught my attention. I would like to use this project for my hand-made arduino oscilloscope project. Unfortunately, you use a Makefile as build system for this project. I think it is good to use CMake instead of Makefile. So I replaced Makefile with CMake. In addition, showing how demo program looks like on README.md is eye-catchy and facilitate some one to use this project for their project. Thus, I added snapshot of demo program to README.md.

Hope this pull request helpful for you.

Thank you.

[SwiTool]

Oh thank you, I downloaded and compiled it to see how it actually looked like. Sad I did not see your PR before 😦