Rendering just edges and vertices, independent of faces

[Mokshit06]

This PR adds the ability to render edges and vertices in fold files that don't necessarily have faces defined by decoupling the vertices & edges from faces in data structure of model. Takes #47 as the starting point. Should close #10

[Mokshit06]

It actually looks like this change makes so that the edges & vertices on a face behind the current one are not "covered"/dimmed by the face above it.
Current behaviour:

Updated behavior:

Maybe we should then keep the previous code for grouping edges & vertices under a face in case faces_vertices are present, and only decouple if they aren't