Standardize on the GL-capable platform plugin

Since the OpenGL classes were moved into QtGui, there's no longer any simple way of detecting whether an app will use OpenGL or just raster. If you try to use OpenGL features without including QtOpenGL or QtQuick, it will fail. There's no way of specifying this manually, so adding the additional dependency is the only work-around (without editing generated files.)

Ideally, we should standardize on a single platform plugin instead of having the two alternatives. This should simplify the code a lot as well. However, we need to first make sure the GL-plugin has the features required for widget apps.