Tiny layer to get OpenGL more structured

**Summary**
Based on community feedback, it is hard for beginners to get started with SharpGL resp. OpenGL. As a user of SharpGL you have to know the OpenGL: where the functions are, how they work, meaning of all the constants and so on. So looking for a particular constant can be pain.

**Acceptance Criteria**
1. OpenGL constants are structured (class/file based) by their semantic meanings. Maybe enums are introduced to structure them.
2. OpenGL functions are structured (class/file based) by their semantic meanings.
3. Code documentation refers down to OpenGL.cs and vice versa.