Editor: Add exntesion method for drawing bonds.

git-tfs-id: [https://tfs.codeplex.com:443/tfs/TFS15]$/Atomix;C40866
Editor: Add constants for bond directions.

git-tfs-id: [https://tfs.codeplex.com:443/tfs/TFS15]$/Atomix;C40867
Editor: Use extension method for drawing.

git-tfs-id: [https://tfs.codeplex.com:443/tfs/TFS15]$/Atomix;C40868