ignore Thumbs.db in workspace-view and in actual atom gitignore

Since you ignore `.DS_Store` I think it's appropriate to ignore `Thumbs.db` in the editor too now since Windows users can also use Atom. These two files are functionally identical between the operating systems and are just clutter.

I also threw Thumbs.db into the gitignore since Windows developers could accidentally commit it when contributing to Atom. (which you might see more of since it's open source now)