add conditional style configurator to all demos

[riri]

I have added the new style-configurator demo to all platform demos, and tested what I could test (on linux).

There are still a few things that could be improved, that why I suggest we keep that branch for further updates:

• add a style section for knobs
• find a way to keep the color table inside the demo function

Other points:

• during testing, I couldn't make sfml_opengl2 and sfml_opengl3 demos working, because they rely on hard coded paths.
• the glfw_opengl4 is segfaulting, even without the addition.

[RobLoach]

I like this, looks great.

Outside the scope of this changeset, but I think it would be really cool to be able to click an Export button to send the contents of the style changes to a .h file in your clipboard.