Add OpenGL example/tool for creating contexts of different versions and profiles

Most examples in qtbase/examples/opengl are pretty ancient. It is high time to add an example that demonstrates requesting GL 2/3/4 contexts with various profiles and options.

This would also be a valueable tool in testing, both during development and for end users, since one would immediately see what kind of context is returned for a given QSurfaceFormat.

The application should be widget based, with a GL-based QWindow embedded. The UI should allow specifying the version, profile and options. The QWindow would then render something while showing also the usual information about the context (GL_VERSION etc.)