Meshdata update

Some modification in meshdata structure and an example with qt backend to generate, cone, cylinder, sphere and arrow


Ok, I am still confused about why this indexing mode is being added to `faces`. It does not appear to be used anywhere in this PR. Generally if you would only return `np.arange`, then there is no need to use a face index at all; the vertexes are already in the correct order to draw with `GL_TRIANGLES`.