Add example program and make this repository compile by itself.

[JohannesWilde]

If I understood it correctly, this repository was meant to be used as a submodule in an other project and therefore the CMakeLists.txt file was kept minimal. I on the other hand wanted to test this repository and check whether it compiles and runs with my current setup [compiler, Qt version]. To that end I would have liked a minimal example.

In addition I was somewhat confused about the "INTERFACE" nature of the "library". If you don't like my changes regarding to that point, simply let me know and I can revert them.

So thanks for this great project. And I would be glad about any feedback.